Obsidian Obsidian By Jim Miller, B.Sc., M.Sc. Geology* Obsidian...even the name is exotic. Ever since I had my first rock collection as a child, I've loved obsidian. Sharp and shiny, obsidian is so different from other rocks. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass. That amazing trip really opened my eyes. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more. My goal in this article is to increase your awareness of some of the more fascinating aspects of this incredible stone. matrix . When I say "crystals," don't visualize those beautiful pointed prisms of quartz found in geodes. All rocks consist of mixtures of various crystalline minerals. When crystallization occurs, the atoms that comprise a mineral become arranged in regular, geometric patterns that are unique to the specific mineral.

Crystal faces form only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free faces. Granite is composed entirely of intergrown crystals of quartz, feldspar, mica and other minerals. These relatively large mineral crystals (easily visible to the naked eye) give granite a rough surface. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic "conchoidal" fracture. This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass. The intersections of conchoidal fracture surfaces can be sharper than a razor. This had obvious advantages for our Stone Age ancestors, who used obsidian extensively for tool making. rhyolite , which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock." It is really a congealed liquid with minor amounts of microscopic mineral crystals and rock impurities. Obsidian is relatively soft with a typical of 5 to 5.5 on the mineral hardness scale. In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0.

Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the of obsidian. Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon. If obsidian is similar in composition to granite and rhyolite, both of which were originally molten, then why is obsidian glassy? The answer relates to the original cooling rate and water content of the magma. Granite cools very slowly miles below the surface of the earth; this slow cooling over millions of years allows for the formation of sizable mineral crystals within the slowly cooling mass of molten rock. Rhyolite typically cools more rapidly near the earth's surface and contains smaller mineral crystals than granite. When rhyolite magma approaches the earth's surface and the pressure of burial decreases, most of the water in the magma is lost as steam.

The resulting silica-rich magma with little remaining water becomes very viscous (thick and pasty) obsidian magma. This magma is so viscous that sizable mineral crystals cannot grow before chilling of the magma "freezes" crystal development. Some obsidian is erupted as lava flows at the ground surface. These surface flows are so viscous that they flow very slowly. One article I read indicated that "an ant could probably outrun an obsidian , a glassy, bubble-rich, lightweight rock that develops when water vapor (steam) escapes rapidly from the molten glass at or near the ground surface. Sometimes obsidian of excellent quality develops as surface lava flows. However, the best quality obsidian often forms below the ground surface around volcanic vents. Silica-rich magma squeezes into rock fractures to form layers and lenses of obsidian that are relatively free of dirt, viscosity of the molten obsidian prevents effective mixing of these magmas, resulting in obsidian that is "streaked" with different layers or colors. Each of these streaks or lines may represent a distinct pulse of an obsidian eruption.

You can visualize the process that results in streaked obsidian if you consider two blobs of green and red taffy (a viscous candy) that are mixed together. Distinct streaks of red and green taffy result as the blobs are mixed. In the case of obsidian, the slow flow of stiff, viscous magma away from the source vent provides the mixing needed to create the layered or streaked varieties of obsidian. The "midnight lace" variety of obsidian often has incredibly contorted streaking, apparently formed as the obsidian layers were stretched and rolled with slow movement of the magma. The various colors of obsidian are a result of several factors. Clear varieties of obsidian contain very few opaque impurities or microscopic mineral crystals. Red or brown obsidian generally results from tiny crystals or inclusions of hematite or limonite (iron oxide). Abundant, microscopic crystals of minerals like magnetite, hornblende, pyroxene, plagioclase and biotite, combined with tiny fragments of rock, likely produce the jet-black varieties of obsidian. Microscopic crystals of various types of feldspars may yield the unique blue, green, purple or bronze colors associated with rainbow obsidian.

The reflectance of rainbow obsidian is likely attributed to a preferred orientation of microscopic crystals of feldspar or mica oriented along flow layers. A certain amount of water always is present in obsidian. Very small inclusions of water vapor in the form of bubbles often are trapped in the glass. Tiny gas bubbles that have been stretched nearly flat along the flow layers in obsidian generally cause the reflectance of gold sheen and silver sheen obsidian. Some of these bubbles are visible to the naked eye. The bubbles can be seen readily with a strong magnifying glass or a microscope. Obsidian is relatively unstable from a geologic perspective. It is rare to find obsidian older than about 20 million years, which is very youthful in comparison to most continental rocks that form the Earth's crust. Over a long period of time, obsidian gradually changes from glass to rock in a process known as "devitrification." In this process, the silica molecules within the glass slowly rearrange into organized crystal patterns. The "snowflakes" in snowflake obsidian are quartz crystals that have formed through devitrification of the original obsidian. The crystals that develop through devitrification cause obsidian to lose its conchoidal fracture and glassy texture.

Native Americans discovered almost all of the obsidian locations in North America. Each obsidian source area has a unique assemblage of trace elements, allowing identification of the original source locality for the obsidian used in prehistoric artifacts. Trace element analyses have shown that native peoples traded this valuable commodity many hundreds of miles from the volcanic source areas along numerous trade routes. The fact that obsidian was transported great distances attests to the mystique of this unique material. My favorite place to collect obsidian is Glass Buttes in central Oregon. A trip to Glass Buttes is well worth the effort for the natural scenery, abundant and beautiful gem-quality obsidian, and exploration of a fascinating geological area. These sagebrush- and juniper-covered hills are remote but accessible by automobile. Located in central Oregon south of Highway 20 and about 80 miles east of Bend, a visitor will be rewarded with many different varieties of obsidian through exploration of the area. Fist-sized pieces are abundant and can be collected with no digging. For large pieces of high quality obsidian (ranging up to several hundred pounds), considerable effort with a shovel and pry bar are necessary.

Further information on Glass Buttes can be found in several rockhound guidebooks that include the central Oregon area. * Jim Miller is a practicing geologist and an avid flint knapper residing in Bothell, Washington. Examples of obsidian from Glass Buttes and stone tools made from those materials by the author are included with this article. Several varieties of obsidian from Glass Buttes, Oregon and tools made by the author from those materials. From left to right across the top, the obsidian varieties are silver sheen, midnight lace, pumpkin and red mahogany. Source material and stone tools of red mahogany obsidian. Source material and stone tools of red mahogany obsidian. Pencil gives scale. Source material and stone tool of silver sheen obsidian. Source material and stone tools of pumpkin obsidian. Pencil gives scale. Multi-colored streaks oriented along flow lines in midnight lace obsidian. Contorted streaks within midnight lace obsidian. viscosity that has the greatest control on the ordered crystalline pattern, the measure of viscosity is dependent on the temperature, crystal content and chemical composition .

Viscosity is a measure on the ability of substance to flow, high viscosity means poor ability to flow and low viscosity means good ability to flow, an example of rhyolite . For obsidian to form, magma is trapped below the eutectic, point of crystallization, by loss of heat. Therefore leaving a magma that is unable to crystallize will form (glass) obsidian. For this process to occur during a lava flow the lava is caught just below crystallization temperature, thus forming a glass due to the inability to form a crystalline solid. The formation of obsidian could also be the melt, liquid remaining from a magma after crystallization, of a rhyolite magma that has been erupted before any crystals can form as stated earlier. The gas content of obsidian is very low so for this to occur the gas has to be released in some way before the dacite . This difference is due to the difference in composition, specifically volatile, gas, content. Volatiles within highly viscous magmas can produce eruptive events due to the inability for the volatiles to escape easily so as they rupture, burst, they release an enormous amount of pressure producing an eruption such as the Mount. St.

Helens eruption on May 18th 1980. For these explosive types of eruptions not occur in the emplacement of obsidian the volatile content for the obsidian must be low. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977). The low amount of water in comparison with rhyolite pumice indicates that the flow of obsidian must take place at the end stage of the explosive eruption phase of rhyolite magma (Bakken Barbara., 1977) end stage indicating after the vent of the volcano has released a large amount of gas through the explosive eruption stage. When it comes down to it the temperature ot the obsidian magma is the may property controling the viscosity over any of the other properties. The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference.

Initially the magma erupts at a temperature around 900 degrees C however this first eruption is rich in volatiles producing pumice, so this stage still has a greater viscosity than obsidian. As volatile as pumice is released and the obsidian melt from the rhyolite is built up at a temperature around 900-700 degrees C is than released as the obsidian at a low viscosity of a magnitude of 10^8 Pa s, which is an order of 3 magnitudes less than that of the original rhyolite pumice eruption, this therefore indicates that the eruption of the obsidian has a similar temperature of eruption as the initial pumice only difference is in the magma's gas content. After obsidian has been erupted what to follow is typically a rhyolite dome rich in crystals due to slow cooling of the magma; this event makes it clear that the reason for lack of crystals in the obsidian are due to being erupted at a temperature greater than the final rhyolite magma.

This information comes to the conclusion that the flow of obsidian is dependent on the time for the obsidian magma melt to cool, and that amount of time for cooling is very short because if allowed to cool to a temperature lower than initial temp it will not flow. Bibliography: Bakken, B., (1977), Obsidian and Its Formation. North West Geology. 6-2, (88-92) Obsidian and its Groovy Flow Emplacement A model for rhyolite composition approaches the surface volatiles exsolve, separate, from the magma and rise to the plumes top. The decrease in the confining pressure leads to frothing of the volatile rich top section of the plume as ascension of the stratified mass occurs. This process of vesiculation leads to some number of explosive eruptions depositing . The transition from the explosive eruption to the pumiceous lava occurs once gas pressure in vesicles can no longer exceed strength of magma.

Later in the eruption following the pumiceous lava flow the event of the emplacement of obsidian occurs, this process happens with the bubble free obsidian flowing out over the earlier emplaced coarsely vesicular pumice (highly inflated pumiceous lava). The Obsidian is slightly less viscous but more dense than the coarse pumice. There remains a gradual transition from the coarse pumice to obisidian due to volatile decrease with the progression from pumice phase to obsidian phase. Essentially while obsidian is erupted vesicular material continues to be extruded. As the obsidian rides over the coarse pumice, volatiles continue to evolve from the cooling upper flow surface of the obsidian flow. This process of the volatiles exsolving from the obsidian flow forms a finely vesicular layer that keeps the flow interior of the obsidian insulated and warm; this contact is also gradational, just as the coarse pumice to the obsidian was.

Within the volcanic vent that the obsidian flowed from the remaining magma from within cools and crystallizes and then erupts as a crystal rich rhyolite flow capping the vent mouth as a rhyolite pyroclastic flow occurred essentially taking the place of the pumiceous lava flow, however there remain deposits of course pumice within the region of Big Obsidian Flow which many indicate a short event of pumiceous flow. The final stage was the emplacement of Big Obsidian Flow that moved very slowly and according to Sherrod et al (1997) this lava flow probably moved a few meters to tens of meters a day. From observations in the field of the Big Obsidian Flow the presence of a thin visculated layer coats some of the surface of the obsidian flow. The final stage of the emplacement of a rhyolite dome is not present the vent is actually plugged by the obsidian flow indicating that the rhyolite stage was not present in the eruption or possibly the rhyolite magma was already to cool and harden to come to the surface of the flow.

This is an image of the Big Obsidian Flow of Newberry Volcano,a USGS Photograph taken in October 1987 by Willie Scott. The structure of the deposit indicates that the obsidian flowed over being erupted explosively. Fink (1983) provided most of the information regarding the flow emplacement model of obsidian as well as the figures displayed. Obsidian occurs as a flow, not as an explosive eruption in contrast to a vesiculated rhoylite pumice or dacite . This difference is due to the difference in composition, specifically gas content. Gases within highly viscous magmas can produce eruptive events due to the inability for the volatiles to escape easily from the magma so as they rupture they release an enormous amount of pressure producing an eruption such as the Mount. St. Helens eruption on May 18th 1980. For these explosive types of eruptions not occur in the emplacement of obsidian the gas content of the obsidian must be low. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977).

The low amount of water in comparison with rhyolite pumice indicates that the flow of obsidian must take place at the end stage of the explosive eruption phase of rhyolite magma (Bakken Barbara., 1977) end stage indicating after the vent of the volcano has released a large amount of gas through the explosive eruption stage. When it comes down to it the temperature ot the obsidian magma is the property controling the viscosity over any of the other properties, higher the temperature the lower the viscosity. The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference. Initially the magma erupts at a temperature around 900 degrees C however this first eruption is rich in volatiles producing pumice, so this stage still has a greater viscosity than obsidian.

As volatile rich pumice is released and the obsidian melt from the rhyolite is built up at a temperature around 900-700 degrees C the obsidian melt can be erupted at a low viscosity of a magnitude of 10^8 Pa s, which is an order of 3 magnitudes less than that of the original rhyolite pumice eruption, this therefore indicates that the eruption of the obsidian has a similar temperature of eruption as the initial pumice only difference is in the magma's gas content. After obsidian has been erupted what to follow is typically a rhyolite dome rich in crystals due to slow cooling of the magma; this event makes it clear that the reason for lack of crystals in the obsidian are due to being erupted at a temperature greater than the final rhyolite magma. This information comes to the conclusion that the flow of obsidian is dependent on the time the obsidian is erupted during the eruption, the temperature of the obsidian and the gas content. Bibliography: Fink, J.H., (1983), Structure and emplacement of a Rhyolite Obsidian Flow: Little Glass Mountain, Medicine Lake Highland, Northern California.

Geological Society of America Bulletin. 94 (362-380) Sherrod., Mastin., Scott., Schilling. (1997). Newberry , Oregon Big Obsidian Flow - Latest Eruptive Event Excerpt from: Sherrod, Mastin, Scott, and Schilling, 1997, Volcano Hazards at Newberry Volcano, Oregon. U.S. Geological Survey Open-File Report, 97(513) The Fly Style of Obsidian Flow Textures and Features Obsidian flows usually consist of fold surfaces, explosion craters, flow banding, and cavities underling the craters. The explosion craters are commonly located on the surface of the flow and can have diameters of 10-25m (36-91ft), at 5-15m (15-55ft) deep; the accompanying cavities can be 5-15m in length. The cavities form near the surface by a buckling mechanism.

Buckling occurs by contrast of stiff upper layer relative to the near liquid flow interior, which forces the upper layer to make an almost triangular shape at the surface, separating it from the lower layer. These cavities can collect /meteoric water*, and as vapor pressure builds up and exceeds the strength of the surface crust of cavity an explosion results producing the explosion caters. *Meteoric water: rain water/ground water, not from the magma . Image depics the event of cavities forming by buckling and exploding due to gas pressure exceeding the strength of the rock. This illustration comes from Castro's Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano . Flow banding in obsidian is a distinctive characteristic and the abundance of the flow banding. Even with the abundance of these flow bands within obsidian little is known about the origins of these features, but what is known is that these freatures derive from both crystallization and deformation processes. According to a paper by J.M. Castro the flow bands develope from the deformation of microlites (very small ) by shear strains during the flow of the obsidian. The picture below shows the flow banding as pointed out by the arrow.

Bibliography Castro, J., Cashman, K., Jaslin, N., Olmsted, B., (2002) Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano. Journal of Volcanology and Geothermal Research. 114 (3-4), (313-330) Castro, J. (2005). New insignts on the origin of flow bands in obsidian. Special Paper - Geological Society of America, 39655-65. doi:10.1130/2005.2396(05). Water Plus Obsidian Obsidian After an obsidian flow has been emplaced it is subject to the atmosphere that causes weathering to the obsidian. One specific type of weathering done on to obsidian is called hydration, which occurs by the water within the atmosphere being absorbed by the obsidian thus increasing the water content within the rock. The product of a hydrated obsidian is called devitrified obsidian, (de- to remove) (vitrify- glassy), so essentially the removal of the glassy property of the obsidian.

The products of devitrified obsidian can produce secondary fiberous mineral crystals that can form ball like shapes called spherulites that are embedded within the obsidian, these secondary minerlas can develope into what is called snowflake obsidian as well, which makes the obsidian look as if it is decorated with snowflackes due to the snowflake shapes the secondary minerals produce. The snowflake obsidian can be used as jewlery and many rockhounds search for it, Yellowstone is home to snowflake obsidian. This is an image of snowflake obsidian, as you can see the seconday minerals group together forming snowflake like shapes scattered across the rock surface. This image is from Rockhoundblog.com. This is an image of what a spherulite looks like due to secondary mineralization from devitrification of obsidian. Photo by (c) 2008 Andrew Alden, licensed to About.com New Research concerning Hydration of Obsidian Some researchers have used the hydration rate of obsidian to acquire specific dates of the obsidian, which is based on the idea that a freshly broken obsidian surface begins to absorb water from its environment almost immediately.

Absorption continues with time, generating a hydrated layer whose thickness is proportional to the time the glass surface was exposed this according to Anovitz et al (2006). This technique of dating can be unreliable due to the many factors influencing rate of hydration. Just recently Anovitz et al discovered that the natural hydration of obsidian can be used as a tool to provide paleoclimatic reconstructions. Anovitz et al (2006) article in GSA’s Geology Journal, “Obsidian hydration: A new paleothermometer,” the first successful application of this idea was done on samples from the the Chalco site in the Basin of Mexico successfully obtaining the temperature change within the region of study, for further information refer to the Anovitz (2006) article. Bibliography: Anovitz, L. (2006). Obsidian hydration; a new paleothermometer. Geology [Boulder], 34(7), 517-520. doi:10.1130/G22326.1. Francis, Peter., and Oppenheimer, Clive.

(2004), Volcanoes, Second Edition. New York: Oxford University Press Inc., New York: (162-164). Obsidian: Igneous Rock - Pictures, Uses, Properties Obsidian What is Obsidian, How Does it Form, and What is it Used For? Obsidian: The specimen shown above is about two inches (five centimeters) across. The curved semi-concentric ridges are breakage marks associated with obsidian's conchoidal fracture. The rock has very sharp edges. What is Obsidian? Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. It is an amorphous material known as a " mineraloid ." The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo). Where Does Obsidian Form? Obsidian is usually an extrusive rock - one that solidifies above Earth's surface.

However, it can form in a variety of cooling environments: along the edges of a lava flow (extrusive) along the edges of a volcanic dome (extrusive) around the edges of a sill or a dike (intrusive) where lava contacts water (extrusive) where lava cools while airborne (extrusive) Types of Obsidian: The specimens shown above are from Glass Butte rockhounding site in central Oregon. It shows the diversity of obsidian types that can be found in a small geographic area. Clockwise from upper left are: double flow obsidian, rainbow obsidian, black obsidian, pumpkin obsidian, mahogany obsidian, gold sheen obsidian, and the piece in the center is gold sheen. The nice photo above is from the Glass Butte Rockhounding Site page on the Deschutes National Forest website . Mahogany obsidian: A tumble-polished specimen of "mahogany obsidian." Image © iStockphoto / Arpad Benedek. What Color is Obsidian? Rainbow Obsidian: A baroque cabochon of iridescent "rainbow obsidian." Black is the most common color of obsidian. However, it can also be brown, tan, or green.

Rarely, obsidian can be blue, red, orange, or yellow. The colors are thought to be caused mainly by trace elements or inclusions. Occasionally two colors of obsidian will be swirled together in a single specimen. The most common color combination is black and brown obsidian swirled together - that's called "mahogany obsidian" (see photo). As a "glass," obsidian is chemically unstable. With the passage of time, some obsidian begins to crystallize. This process does not happen at a uniform rate throughout the rock. Instead it begins at various locations within the rock. At these locations, the crystallization process forms radial clusters of white or gray cristobalite crystals within the obsidian. When cut and polished, these specimens are referred to as "snowflake obsidian" (see photos). Rarely, obsidian has an iridescent or metallic "sheen" caused by light reflecting from minute inclusions of mineral crystals, rock debris, or gas. These colored specimens are known as "rainbow obsidian," "golden obsidian," or "silver obsidian," depending upon the color of the sheen or iridescence. These specimens are very desirable for the manufacture of jewelry.

Snowflake obsidian: A tumble-polished specimen of "snowflake obsidian." Image © iStockphoto / Martin Novak. What is the Composition of Obsidian? Most obsidians have a composition similar to rhyolite and granite . Granites and rhyolites can form from the same magma as obsidian and are often geographically associated with the obsidian. Rarely, volcanic glasses are found with a composition similar to basalt and gabbro . These glassy rocks are named "tachylyte." Are There Other Glassy Igneous Rocks? Pumice , scoria , and tachylyte are other volcanic glasses formed by rapid cooling. Pumice and scoria differ from obsidian by having abundant vesicles - cavities in the rock produced when gas bubbles were trapped in a solidifying melt. Tachylyte differs in composition - it has a composition similar to basalt and gabbro. Obsidian outcrop: Obsidian along the edge of a lava flow in central Oregon. Image © iStockphoto / Phil Augustavo. Obsidian knife blade: A knife blade manufactured from mahogany obsidian.

The craftsman who made this blade had a very high skill level and was able to produce a serrated edge. Image © iStockphoto / Al Braunworth. Occurrence of Obsidian Obsidian is found in many locations worldwide. It is confined to areas of geologically recent volcanic activity. Obsidian older than a few million years is rare because the glassy rock is rapidly destroyed or altered by weathering, heat, or other processes. Significant deposits of obsidian are found in Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Russia, United States, and many other locations. In the United States it is not found east of the Mississippi River, as there is no geologically recent volcanic activity there. In the western US it is found at many locations in Arizona, California, Idaho, Nevada, New Mexico, Oregon, Washington, and Wyoming. Most obsidian used in the jewelry trade is produced in the United States. Obsidian spear point: A spear point fashioned from opaque black obsidian. Image © iStockphoto / Charles Butzin. Rock & Mineral Kits: Get a rock, mineral, or fossil kit to learn more about Earth materials.

The best way to learn about rocks is to have specimens available for testing and examination. Uses of Obsidian as a Cutting Tool The conchoidal fracture of obsidian causes it to break into pieces with curved surfaces. This type of fracturing can produce rock fragments with very sharp edges. These sharp fragments may have prompted the first use of obsidian by people. The first use of obsidian by people probably occurred when a sharp piece of obsidian was used as a cutting tool. People then discovered how to skillfully break the obsidian to produce cutting tools in a variety of shapes. Obsidian was used to make knives, arrowheads, spear points, scrapers, and many other weapons and tools. Once these discoveries were made, obsidian quickly became the raw material of preference for producing almost any sharp object. The easy-to-recognize rock became one of the first targets of organized "mining." It is probably a safe bet that all natural obsidian outcrops that are known today were discovered and utilized by ancient people. Apache tears: "Apache Tears" is a name used for small obsidian nodules of about one inch or less that can be found in volcanic areas of the southwestern United States. Their name comes from a Native American legend.

During a battle between Apaches and the U.S. Cavalry in 1870, the outnumbered Apaches, facing defeat, rode their horses over a cliff rather than allow themselves to be killed by their enemy. Upon hearing the story of the battle, the tears of their family members turned to stone when they hit the ground. Those stones are now found as the black obsidian nodules. People who do rock tumbling often polish Apache Tears. They are difficult to polish because the obsidian chips and bruises easily. Success occurs when they are cushioned during the tumbling with smaller pieces of rough or small ceramic media. Stone Age Manufacturing and Trade The manufacture of obsidian tools by humans dates back to the Stone Age. At some locations, tons of obsidian flakes reveal the presence of ancient "factories." Some of these sites have enough waste debris to suggest that many people labored there for decades producing a variety of obsidian objects.

Making arrowheads, spear points, knife blades, and scrapers from obsidian, chert , or flint might have been the world's first "manufacturing industry." Obsidian was so valued for these uses that ancient people mined, transported, and traded obsidian and obsidian objects over distances of up to a thousand miles. Archaeologists have been able to document the geography of this trade by matching the characteristics of obsidian in outcrops with the characteristics of obsidian in cutting tools. A study done by the Idaho National Laboratory used composition studies by X-ray fluorescence to identify the source outcrops of obsidian artifacts and map their use across the western United States. Obsidian in Modern Surgery Although using a rock as a cutting tool might sound like "stone age equipment," obsidian continues to play an important role in modern surgery. Obsidian can be used to produce a cutting edge that is thinner and sharper than the best surgical steel. Today, thin blades of obsidian are placed in surgical scalpels used for some of the most precise surgery. In controlled studies, the performance of obsidian blades was equal to or superior to the performance of surgical steel.

Obsidian jewelry: Mahogany obsidian and snowflake obsidian cabochons set in sterling silver pendants. Obsidian for opal triplets: A thin piece of obsidian is often used as a "backing" material for opal doublets and triplets. The black obsidian adds stability to the opal and provides a dark background color that contrasts with the opal's fire. Uses of Obsidian in Jewelry Obsidian is a popular gemstone . It is often cut into beads and cabochons or used to manufacture tumbled stones. Obsidian is sometimes faceted and polished into highly reflective beads. Some transparent specimens are faceted to produce interesting gems. The use of obsidian in jewelry can be limited by its durability. It has a hardness of about 5.5 which makes it easy to scratch. It also lacks toughness and is easily broken or chipped upon impact. These durability concerns make obsidian an inappropriate stone for rings and bracelets. It is best suited for use in low-impact pieces such as earrings, brooches, and pendants. Obsidian is also used in making opal doublets and opal triplets .

Thin slices or chips of opal are glued to a thin slice of obsidian to make a composite stone. The black obsidian provides an inexpensive and color-contrasting background that makes opal's colorful fire much more obvious. It also adds mass and stability to the opal that facilitates cutting it into a gem. Other Uses of Obsidian Freshly broken pieces of obsidian have a very high luster. Ancient people noticed that they could see a reflection in obsidian and used it as a mirror. Later, pieces of obsidian were ground flat and highly polished to improve their reflective abilities. Obsidian's hardness of 5.5 makes it relatively easy to carve. Artists have used obsidian to make masks, small sculptures, and figurines for thousands of years. Obsidian | Define Obsidian at Dictionary.com obsidian [uh b-sid-ee-uh n] /əbˈsɪd i ən/ Spell noun 1. a volcanic glass similar in composition to granite, usually dark but transparent in thin pieces, and having a good conchoidal fracture.

Origin of obsidian Latin 1350-1400 1350-1400; < Latin Obsidiānus, printer's error for Obsiānus pertaining to Obsius, the discoverer (according to Pliny) of a similar mineral in Ethiopia; replacing Middle English obsianus < Latin; see -an Dictionary.com Unabridged Examples from the Web for obsidian Expand Contemporary Examples “I think the main reason is the availability of information and community groups that the Internet provides,” said obsidian. Jack at Sea George Manville Fenn Agates, cornelians, obsidian, are also among the products of this nature. Mexico Charles Reginald Enock British Dictionary definitions for obsidian Expand a dark volcanic glass formed by very rapid solidification of lava Also called Iceland agate Word Origin C17: from Latin obsidiānus, erroneous transcription of obsiānus (lapis) (stone of) Obsius, the name (in Pliny) of the discoverer of a stone resembling obsidian Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd.

1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Word Origin and History for obsidian Expand n. "dark, hard volcanic rock," 1650s, from Latin obsidianus, misprint of Obsianus (lapis) "(stone) of Obsius," name of a Roman alleged by Pliny to have found this rock in Ethiopia. Online Etymology Dictionary, © 2010 Douglas Harper Obsidian Rock Facts Obsidian Rock Facts Obsidian Rock Facts Obsidian is an igneous rock that forms when molten rock cools very rapidly. The result is a rock that cooled so fast, crystals did not get a chance to form. Obsidian is a volcanic glass with a smooth and uniform structure. Interesting Obsidian Rock Facts: Different varieties of obsidian rock get their names from their appearance. The appearance of obsidian is based on the chemical composition of the magma that cooled to make the rock.

Obsidian is also known as "nature's glass" because of its glassy appearance. When obsidian breaks, the fractures are very sharp which is why it was used as tools in the Stone Age. Seventy percent of obsidian rock is made of silica. Obsidian can only form near active volcanoes. Sometimes obsidian is erupted directly from a volcano. The best obsidian is formed underground near a volcanic vent because those rocks will have limited impurities. Obsidian flows are so slow that often other obsidian flows will occur on top of each other causing a streaked appearance in the obsidian rock. Tiny gas bubbles that trap water are a part of every obsidian rock. Although most rocks that form in the Earth's crust are very old, obsidian is rarely older than 20 million years old which is young for a rock. Obsidian goes through a process called devitrification whereby it turns from glass to a rock. Obsidian is commonly used during surgical procedures because it is often sharper than traditional surgical tools. Obsidian is used as a gemstone in jewelry. Related Links: Obsidian | Volcano World | Oregon State University OREGON STATE UNIVERSITY By Jim Miller, B.Sc., M.Sc.

Geology* Obsidian...even the name is exotic. Ever since I had my first rock collection as a child, I've loved obsidian. Sharp and shiny, obsidian is so different from other rocks. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass. That amazing trip really opened my eyes. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more. My goal in this article is to increase your awareness of some of the more fascinating aspects of this incredible stone. matrix . When I say "crystals," don't visualize those beautiful pointed prisms of quartz found in geodes. All rocks consist of mixtures of various crystalline minerals. When crystallization occurs, the atoms that comprise a mineral become arranged in regular, geometric patterns that are unique to the specific mineral. Crystal faces form only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free faces.

Granite is composed entirely of intergrown crystals of quartz, feldspar, mica and other minerals. These relatively large mineral crystals (easily visible to the naked eye) give granite a rough surface. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic "conchoidal" fracture. This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass. The intersections of conchoidal fracture surfaces can be sharper than a razor. This had obvious advantages for our Stone Age ancestors, who used obsidian extensively for tool making. rhyolite , which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock." It is really a congealed liquid with minor amounts of microscopic mineral crystals and rock impurities. Obsidian is relatively soft with a typical of 5 to 5.5 on the mineral hardness scale. In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0. Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica.

Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the of obsidian. Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon. If obsidian is similar in composition to granite and rhyolite, both of which were originally molten, then why is obsidian glassy? The answer relates to the original cooling rate and water content of the magma. Granite cools very slowly miles below the surface of the earth; this slow cooling over millions of years allows for the formation of sizable mineral crystals within the slowly cooling mass of molten rock. Rhyolite typically cools more rapidly near the earth's surface and contains smaller mineral crystals than granite. When rhyolite magma approaches the earth's surface and the pressure of burial decreases, most of the water in the magma is lost as steam. The resulting silica-rich magma with little remaining water becomes very viscous (thick and pasty) obsidian magma.

This magma is so viscous that sizable mineral crystals cannot grow before chilling of the magma "freezes" crystal development. Some obsidian is erupted as lava flows at the ground surface. These surface flows are so viscous that they flow very slowly. One article I read indicated that "an ant could probably outrun an obsidian viscosity of the molten obsidian prevents effective mixing of these magmas, resulting in obsidian that is "streaked" with different layers or colors. Each of these streaks or lines may represent a distinct pulse of an obsidian eruption. You can visualize the process that results in streaked obsidian if you consider two blobs of green and red taffy (a viscous candy) that are mixed together. Distinct streaks of red and green taffy result as the blobs are mixed. In the case of obsidian, the slow flow of stiff, viscous magma away from the source vent provides the mixing needed to create the layered or streaked varieties of obsidian. The "midnight lace" variety of obsidian often has incredibly contorted streaking, apparently formed as the obsidian layers were stretched and rolled with slow movement of the magma. The various colors of obsidian are a result of several factors.

Clear varieties of obsidian contain very few opaque impurities or microscopic mineral crystals. Red or brown obsidian generally results from tiny crystals or inclusions of hematite or limonite (iron oxide). Abundant, microscopic crystals of minerals like magnetite, hornblende, pyroxene, plagioclase and biotite, combined with tiny fragments of rock, likely produce the jet-black varieties of obsidian. Microscopic crystals of various types of feldspars may yield the unique blue, green, purple or bronze colors associated with rainbow obsidian. The reflectance of rainbow obsidian is likely attributed to a preferred orientation of microscopic crystals of feldspar or mica oriented along flow layers. A certain amount of water always is present in obsidian. Very small inclusions of water vapor in the form of bubbles often are trapped in the glass. Tiny gas bubbles that have been stretched nearly flat along the flow layers in obsidian generally cause the reflectance of gold sheen and silver sheen obsidian. Some of these bubbles are visible to the naked eye. The bubbles can be seen readily with a strong magnifying glass or a microscope. Obsidian is relatively unstable from a geologic perspective.

It is rare to find obsidian older than about 20 million years, which is very youthful in comparison to most continental rocks that form the Earth's crust. Over a long period of time, obsidian gradually changes from glass to rock in a process known as "devitrification." In this process, the silica molecules within the glass slowly rearrange into organized crystal patterns. The "snowflakes" in snowflake obsidian are quartz crystals that have formed through devitrification of the original obsidian. The crystals that develop through devitrification cause obsidian to lose its conchoidal fracture and glassy texture. Native Americans discovered almost all of the obsidian locations in North America. Each obsidian source area has a unique assemblage of trace elements, allowing identification of the original source locality for the obsidian used in prehistoric artifacts. Trace element analyses have shown that native peoples traded this valuable commodity many hundreds of miles from the volcanic source areas along numerous trade routes. The fact that obsidian was transported great distances attests to the mystique of this unique material. My favorite place to collect obsidian is Glass Buttes in central Oregon.

A trip to Glass Buttes is well worth the effort for the natural scenery, abundant and beautiful gem-quality obsidian, and exploration of a fascinating geological area. These sagebrush- and juniper-covered hills are remote but accessible by automobile. Located in central Oregon south of Highway 20 and about 80 miles east of Bend, a visitor will be rewarded with many different varieties of obsidian through exploration of the area. Fist-sized pieces are abundant and can be collected with no digging. For large pieces of high quality obsidian (ranging up to several hundred pounds), considerable effort with a shovel and pry bar are necessary. Further information on Glass Buttes can be found in several rockhound guidebooks that include the central Oregon area. * Jim Miller is a practicing geologist and an avid flint knapper residing in Bothell, Washington. Examples of obsidian from Glass Buttes and stone tools made from those materials by the author are included with this article. Several varieties of obsidian from Glass Buttes, Oregon and tools made by the author from those materials. From left to right across the top, the obsidian varieties are silver sheen, midnight lace, pumpkin and red mahogany. Source material and stone tools of red mahogany obsidian.

Source material and stone tools of red mahogany obsidian. Pencil gives scale. Source material and stone tool of silver sheen obsidian. Source material and stone tools of pumpkin obsidian. Pencil gives scale. Multi-colored streaks oriented along flow lines in midnight lace obsidian. Contorted streaks within midnight lace obsidian. Obsidian | Gem5.com Obsidian Photo of Obsidian in Processed & Rough Form Obsidian is a mineral with a hardness of 5 out of 10 on the Mohs scale of mineral hardness [ ? ]. These Amorphously structured gems are made of silicon dioxide, their full chemical compound being SiO2. Obsidian is an extrusive igneous rock, a volcanic glass formed by rapid cooling. Its essential component is glass. Its accessory components are magnetite, ilmenite, oxides and other minerals, such as feldspar. It has the same composition as dacite or rhyolite, with less than 1 percent water. Its color is shiny black, with clear conchoidal (shell-like)fracture, its texture glassy with rare microphenocrysts and abundant opaque fine dust scattered in concentric zones.

The black color appears because of the way the embryonic crystals in the glass scatter the light. Varieties show golden or silver sheen caused by inclusions. Obsidian may also be red-brown due to the presence of iron, and greenish or grey due to the presence of tiny gas bubbles. It has a massive structure, with, in some cases, the presence of latent concentric fractures (perlitic fractures). Obsidian is derived from the rapid cooling of a viscous, granitic magma. It is found as volcanic units, as fragments thrown into the air by erupting volcanos, sometimes as long filaments ("Pele's hair") and as the outer covering of rhyolitic and dacitic domes . It is not widely distributed, although it is the commonest of natural glasses. But it is very common in recent lava flows. mostly from felsic volcanos (Japan, Java). It is found in the tertiary lavas of the Lipari Isles (Italy). The most striking occurrence is in Obsidian Cliff in Yellowstone National Park, Wyoming (USA). It is also found in Hungary and Iceland. The specific gravity [ ? ] for Obsidian is 2.35, it's refractive index [ ?

] is 1.48-1.51, and it's double refraction [ ? ] is None. History Obsidian is named after a Roman, "Obsius." Shattered fragments of obsidian have sharp edges high vitreous luster, and was used in prehistoric times and antiquity for tools and sculptures. Industrial Usages It is currently used industrially as raw material for rock wool. Especially prized are such varieties as snow-flake obsidian, which contains small, grey spherullites, and the silky lustred obsidian of Mexico. You May Also Like... Tektites: Tektites are glass specimens rich in silica that many believed were actually meteorites. But, the locations in which tektites have been found, and their chemistry, led scientists to suggest that they may not in fact have come from outside planet Earth. Another conjecture is that tektites are formed from rocks that melted after being hit by a meteorite.

Tektites actually have a composition not unl ( read full ) Onyx: Onyx is a translucent to semitransparent variety of Agate, which in turn is a variety of Chalcedony or cryptocrystalline quartz that is porous and consists of straight or parallel banded agate of alternating shades of milky white and black, gray and black, black and red, white and red, white and brown. The term onyx has been erroneously applied as a suffix to Brazilian onyx, Mexican onyx, Orient ( read full ) Tags Obsidian types, their descriptions and photos I actually loved this picture. The description might have read. Under an Aurora borealis, lightning struck a mineral laden vug and created this once in the lifetime gem. BID NOW while it is 9.99 instead of 999.99 Be sure to leave feedback after doing some tests. This was one lucky fella to get these two rare pieces from one unknown source. WOW! As I posted to the eBay seller, I have never seen glass clear brightly colored obsidian!

Only clear brown, gray clear apache tears obsidian and some of the clear flow obsidians with the flames of mahogany (ribbon obsidian) There are some sellers on the bay selling green and purple glass as obsidian. Lots of gimicks out there that will lighten your wallet and tell you a lie! There are obsidians that are deep purple, deep green and yellow that have translucents but they are VERY rare. Latin Obsidiï¿½us, printer's error for Obsinus pertaining to Obsius, the discoverer (according to Pliny) of a similar mineral in Ethiopia; compare Middle English obsianus Obsidian is volcanic glass. It often has flow lines that create interesting effects. Gold sheen and silver sheen obsidian's display gold and silver shimmerings when polished, rainbow obsidian displays different colors, etc. Obsidian has a reputation for being difficult to polish. Usually the reason for difficult polishing is that coarse grits can create a fairly deep fracture zone that doesn't come out when the stone is cut at finer grits. Another problem is obsidian doesn't polish well if it gets too hot.

So, if you don't use super coarse grinding wheels and if you keep your polishing wheel slow and wet, good results will easily be obtained. Diamond wheel finishing works well. Usually Obsidian is a jet black stone with a vitreous (glassy) lustre. Just like glass, obsidian tends to shatter into sharp fragments when hit hard, this strength and brittleness have resulted in its use as sharp cutting implements and weapons such as spear and arrowheads or inserted into wood made wonderful clubs. Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color. Very few samples are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals of quartzlike cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian) or an iridescent, rainbow-like sheen (rainbow obsidian).

Mahogany My own collection has been found and collected in Oregon. When so swirled it can also be called Red Ribbon Black Obsidian Quite rare. Outstanding nice swirls red ribbon black obsidian. THIS IS GEM QUALITY OBSIDIAN FROM GLASS BUTTE, IN CENTRAL OREGON. THIS OBSIDIAN CAN BE KNAPPED INTO BEAUTIFUL SPEAR, ARROW, KNIFE, OR SPEAR POINTS AND CAN ALSO BE CABBED INTO BEAUTIFUL CABS Impurities in obsidian change its appearance; for example, iron may cause it to appear reddish brown. Mahogany obsidian may range from an orangey to reddish brown, mottled with uneven splashes or streaks of black. Pumpkin obsidian -- also a Mahogany type. Pale color and questionably attractive. Fireworks Obsidian (Or Firecracker Obsidian) or fire cracker orbicular mexican obsidian Fireworks Obsidian usually comes from Mexico It takes a good polish and cuts easily but is not suitable for knapping. As with all rocks from nature, there may be fracture lines either visible or invisible. Be aware of this in buying. BOOM!

Light pink to dark red are the colors of these distinct exploding bursts on display against the backdrop of midnight black obsidian.

Question: Obsidian is a form of?
Answer: Rock