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. Cochise College P Brief Introduction to Arrowheads Man has been making tools and weapons out of materials found on our planet ever since we started walking over a million years ago. Primitive Man had few resources to work with at the time; some of these were rocks, bones and wood. Out of these three, stone tools became the most durable and withstand time the best. One of the weapons made from stone are points also known as Arrowheads.

This paper will talk about some of the materials being used and will give some examples of these materials. What makes a great Arrowhead is a hard rock or mineral above 5.5 on Mohs scale of hardness; for example, cryptocrystalline quartz breaks with a sharp conchoidal fracture which is a curved or shell like fracture. Flakes from these fractures can be thin and very sharp. Here are two examples of conchoidal fracturing and flaking, also known as chip or chipping. FLINT GLASSY ROCKS OBSIDIAN (below) is a very fine grained silica-rich volcanic rock with glass as its main component; it is an extrusive igneous volcanic rock that is formed by the very rapid cooling of viscous acid lava. Obsidian is usually black but can range in many different colors like dark olive-green and root-beer smoky; it also can range from transparent to opaque as shown in the examples below. Obsidian is rated 5 to 5.5on Mohs scale of hardness.

MAHOGANY OBSIDIAN (below) in brown- and blacked-streaked ï¿½mahoganyï¿½ and ï¿½midnight laceï¿½ obsidian (not shown here) swirls and streaks are formed as the cooling lava rolled over and over as iron oxides are mixed in, which gives the obsidian its reddish to brownish colors in the streaks and swirls. Obsidian is the sharpest of all the materials being covered here because it is naturally formed glass and, when chipped, the conchoidal fractures can be very thin and extremely sharp, like a razor blade. QUARTZ CHALCEDONY (SiO2 Silicon Dioxide) is when quartz forms at low temperatures in volcanic cavities; its crystals can be so small that the mineral looks like porcelain. Chalcedony is the general name for ï¿½cryptocrystalline quartzï¿½. It comes in an astonishing array of colors and patterns, including blood-red carnelian, wine-red jasper, brown-banded agate, green-moss agate, apple-green chrysoprase, and black and white onyx. Chert is also a form of chalcedony.

JASPER (below) Burgundy-wine colored arrowhead, yellow-brown jasper chipping stone. Yellow-red colored jasper scraper from petrified wood, AZ. AGATE (below) Opal agate and a moss-green agate arrowhead BIOCHEMICAL ROCKS are formed when countless creatures are able to extract dissolved chemicals from seawater and use them to make shell and bone. Some use calcium and carbon to make carbonates, others use dissolved silica to make silicates. When these creatures die, the solid material they create turns into sediments, which form ï¿½biochemicalï¿½ sedimentary rocks such as chert, flint, chalk and diatomaceous earth. Prehistoric man used chert and flint because of the ability to conchoidal fracture, great for making tools with. Both chert and flint are opaque and have a waxy to glassy appearance. CHERT (below) is a form of chalcedony and has a hardness of 7 and above on Mohs scale of hardness. Chert is a light tan to brown in color. Igneous Rocks Igneous Rocks Mineral information is presented here; Pictures of minerals are found at the bottom of the page.

Basalts are dark colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior. Granite is an igneous rock that is composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye. Look at the photo of granite above, notice the different crystals in the rock. Granite is an excellent material for building bridges and buildings because it can withstand thousands of pounds of pressure. It is also used for monuments because it weathers slowly. Engravings in the granite can be read for hundreds of years, making the rock more valuable. Granite is quarried in many places in the world including the United States.

The State of New Hampshire has the nickname "Granite State" because of the amount of granite in the mountains of that beautiful state. The Canadian Shield of North America contains huge outcroppings (surface rocks) of granite. is an extrusive igneous rock. The principle minerals that make up dacite are plagioclase, quartz, pyroxene, or hornblende. cools very quickly. The lava coo ls so quickly that no crystals can form. When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way. Obsidian is usually black or a very dark green, but it can also be found in an almost clear form. Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds. Today obsidian is used as a scalpel by doctors in very sensitive eye operations. Gabbro is a dark-colored, coarse-grained intrusive igneous rock. Gabbro is very similar to basalt in its mineral make up. It is composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine.

Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite, giving it a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende. An Example of Igneous Rocks An Example of Igneous Rocks The following is an example of igneous rocks. It is not an all inclusive list but a brief pictorial list of some common igneous rocks. Igneous rocks get their name from the latin word for fire “igneus”. The name is appropriate because these rocks are born of fire. Beneath the thin rocky crust of the earth is the inferno of the mantle! The mantle is the origin of this rock type. Andesite is a gray to black volcanic rock. It is generally erupted from stratovolcanoes as thick lava flows. It can also generate strong explosive eruptions to form pyroclastic flows. Andesites erupt at temperatures between 900 and 1100° C.

Andesite contains crystals composed of plagioclase feldspar, pyroxenes, and hornblende. The word andesite comes from the Andes Mountain Range where andesite is common. Basalt Lava - Basalt is a hard, black volcanic rock. Less than ½ of the weight of basalt is silica (SiO2). Because of basalt's low silica content, it has a low viscosity (resistance to flow). This enables basaltic lava to flow quickly and allows volcanic gases to escape without explosive events. The minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 to 1250° C. Basalt is the most common rock type in the Earth's crust most of the ocean floor is made of basalt. Basaltic magma is commonly produced by direct melting of the Earth's mantle Dacite lava is most often light gray, but can be dark gray to black. It is one of the most common rock types associated with enormous Plinian-style eruptions. Dacite lava consists of about 63 to 68 percent silica (SiO2).

Dacite generally erupts at temperatures between 800 and 1000°C Common minerals include plagioclase feldspar, pyroxene, and amphibole Dacite has a high crystal content. This combined with a high silica content makes it viscous and prone to explosive eruptions. Dacite was erupted from Mount St. Helens 1980-86 Pumice is light and porous. It forms during explosive eruptions. Pumice is full of holes caused by expanding volcanic gases. It is composed of volcanic glass and minerals, and can form in all types of magma: basalt, andesite, dacite, and rhyolite. Obsidian is usually black in color though it can also be red or have a greenish tint. It is a dense volcanic glass, usually composed of rhyolite, rich in iron and magnesium. Obsidian is formed when the lava cools so quickly that crystals do not have time to grow. Obsidian fractures with very sharp edges. It was used by Stone Age cultures for making knives, arrowheads, and other tools where sharp edges are important. Rhyolite is a light-colored volcanic rock.

It has a high silica content which makes it very viscous. This prevents gases from escaping causing rhyolite eruptions to be explosive. Rhyolite has a silica (SiO2) content greater than about 68 percent by weight. Common mineral types include quartz, feldspar and biotite Rhyolite is erupted at temperatures of 700 to 850° C. Rhyolite eruptions often produce pumice or obsidian All pictures on this page are courtesy of the USGS - United States Geological Survey. Discriptions are adapted from the USGS. Igneous Rocks | Volcano World | Oregon State University OREGON STATE UNIVERSITY Mineral information is presented here; Pictures of minerals are found at the bottom of the page. Basalts are dark colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior.

Granite is an igneous rock that is composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye. Look at the photo of granite above, notice the different crystals in the rock. Granite is an excellent material for building bridges and buildings because it can withstand thousands of pounds of pressure. It is also used for monuments because it weathers slowly. Engravings in the granite can be read for hundreds of years, making the rock more valuable. Granite is quarried in many places in the world including the United States. The State of New Hampshire has the nickname "Granite State" because of the amount of granite in the mountains of that beautiful state. The Canadian Shield of North America contains huge outcroppings (surface rocks) of granite. is an extrusive igneous rock. The principle minerals that make up dacite are plagioclase, quartz, pyroxene, or hornblende. cools very quickly.

The lava coo ls so quickly that no crystals can form. When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way. Obsidian is usually black or a very dark green, but it can also be found in an almost clear form. Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds. Today obsidian is used as a scalpel by doctors in very sensitive eye operations. Gabbro is a dark-colored, coarse-grained intrusive igneous rock. Gabbro is very similar to basalt in its mineral make up. It is composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine. Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite, giving it a glassy appearance.

The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende. GemRocks: Obsidian ( Fr- obsidienne/obsidiane; Ger- Obsidian/Feuerkiesel; Nor- obsidian; Rus- ) OBSIDIAN A. Mahogany obsidian (height - 7 cm), also called "Mountain mahogany," from the Jemez Mountains, New Mexico. (© photo by W. Sacco) B. Snowflake obsidian (height - ca. 3.6 cm) from Black Rock, Milford, Utah. (© original photo [here irregularly cropped] by Frederick H. Pough) C. Rainbow obsidian heart (width - 9 cm) from Mexico. Talisman Trading. (© photo by Jeffrey A. Scovil) D. Obsidian hippopotamus (length - ca. 9 cm). Carving from brecciated obsidian by Gerd Dreher. Silverhorn. (© photo by Jeffrey A. Scovil) E. Gold-sheen obsidian (height - 8 cm) from Mexico. Carving appears black when viewed from some directions. Rick Dietrich collection.

(© photo by Dick Dietrich) DESCRIPTION: Obsidian is natural glass formed by quenching (i.e., rapidly cooling) magma of granitic/rhyolitic or similar composition. Colors - commonly dark gray to nearly black, less commonly bluish gray or reddish brown and rarely with streaks of pastel hues that are pinkish, yellowish, greenish, purple, brown, etc.; some is iridescent, most commonly exhibiting silver or golden tones; some is mottled or roughly banded, the latter apparently representing flow patterns. H. ~ 5-5½ S.G.

2.3-2.6 Light transmission - most is transparent to subtranslucent in thin slivers; some reddish brown obsidian and a dark green glass, most of which is sideromelane rather than obsidian (see REMARKS) is virtually opaque Luster - vitreous or subvitreous Breakage - conchoidal fracture is typical Miscellany - some obsidian appears to be chatoyant, iridescent, or aventurescent because of the presence of minute inclusions of minerals -- e.g., hematite and/or ilmenite -- and/or bubbles that are relatively abundant in some lamellae and less so or virtually lacking in intervening lamellae. Streak -- i.e., powder -- of virtually all obsidians, even those that re nearly black, appears white. OTHER NAMES: A few obsidian bodies have extents that have led to their being named according to the scheme used for stratigraphic units (see Appendix B, Glossary). Three examples are the East Lake Obsidian of Obsidian cliff, Yellowstone National Park, Wyoming; the Mono Craters Obsidian of the Sierra Nevada of California; and the Newberry Obsidian of Oregon.

Additional names, many of which have been given in the markeplace, follow: Agata Negra (black agate) - misnomer sometimes used in the marketplace. Apache tears - dark gray to nearly black, pebble-sized glass nodules, most of which have greatest dimensions ranging between two and four centimeters, that occur as remnants within or weathered out of light gray perlite. (Perlite, apparently derived from obsidian as the result of hydration involving meteoric water, is a light gray rock made up of concentrically fractured fragments.) Many apache tears in the marketplace are from Maricopa and Pinal counties, Arizona. Banded obsidian - the banding commonly exhibits a flowlike appearance; some roughly banded obsidian has been incorrectly designated onyx obsidian. Black lava glass - descriptive name given to some obsidian. Colombianite (americanite) - a variety of obsidian from the vicinity of Cali, Colombia. Chatoyant obsidian - name sometimes applied to varieties that are iridescent in diverse colors; appearance is apparently due to the presence of minute inclusions and/or bubbles.

Desert glass - name sometimes applied a pale yellow glass that some people have identified as obsidian but seems more likely to be an impactite glass-- e.g., that from Libya. This term is further confounded because it also has been applied to the amethyst-colored man-made glass frequently found in sandy areas, such as deserts and on beaches, with its purple color apparently developed as a result of exposure of essentially colorless glass to ultraviolet or cosmic rays from the sun. See TECTITE entry. Electric blue obsidian - obsidian with a vibrant blue color. Fire obsidian - iridescent obsidian from Glass Buttes, Oregon; "fire" is said to be due to reflection of light off thin layers the refractive indices of which are higher than the rest of the obsidian because of their containing extremely small (nanometric) magnetite crystals (Rossman and Miller, 2007).

Flame obsidian - another name of fire obsidian: (also called "mar-chett" according to Emory Coons, (p.c., 2008), who also notes that the one from Glass Buttes apparently owes its appearance to "a paper thin opake (sic) line of red mostly[,] sometimes yellow or greenish or mixe of the varying color [and] it is very rare to have what is called fire on it.") Flowering obsidian - similar to snowflake obsidian. Glass (or glassy) lava - overall term that has been used, especially in the field. Glass agate - atrocious misnomer sometimes given obsidian. Golden sheen obsidian (also gold or golden obsidian) - name widely given in the marketplace to sheen obsidian (q.v.) with the precominant "sheen" a golden brown color (see Fig. E). Iceland agate (or Iceland agate lava) - misnomer sometimes given to a brownish or grayish variety of obsidian from Iceland. Iris obsidian - another name for rainbow obsidian. Itatli - obsidian (Aztec). Iztli - obsidian (Aztec).

Lassenite - glass of trachytic composition (trachyte is the aphanitic equivalent of syenite, which is composed of 90 or more percent of alkali feldspar) from the vicinity of Lassen Peak, California. Libyan glass - see Desert glass. Mahogany obsidian (also Mountain mahogany) - reddish subtranslucent obsidian, commonly including black or gray bandlike or streaked swirl-like patterns. See Fig. A. Marekanite - name for Apache tearlike masses OR, according to some ambiguous statements, a mottled brown and black obsidian from the vicinity of the Marekanka River, which flows into the Sea of Okhotsk, off eastern Siberia. Marskanite - name applied variously -- e.g., to mottled brown and black obsidian from Siberia; to any cloudy, smoky gray obsidian; and to brown and gray, commonly in part yellowish or reddish, obsidian (especially those from Mexico). Mexican 'Mayan' - see Rainbow obsidian. Mount Saint Helens emerald - a misleading misnomer given obsidian (and even more often to a green glass produced by melting ash erupted by Mount St.

Helens. Montana jet - a misleading misnomer given some black obsidian from the Yellowstone National Park area, Wyoming. Mountain mahogany - See Mahogany obsidian. Nevada diamond - term that has been applied, albeit rarely, to what is said to be artificially decolorized obsidian. Obsidian "cat's-eye" (or "cat's eye obsidian) - name sometimes given to obsidian that has a golden chatoyant-like appearance; see Golden sheen obsidian. Obsidianite - name introduced by Walcott (1898) for what are now known to be tektites (australites) from Australia; however, according to Bates and Jackson (1987) this term came to have wider usage, and "Most stones originally described as 'obsidianite' were later shown to be true obsidian and not tektites." [To date, however, I have not found examples to document this purported later, wider usage.] Onyx obsidian - term sometimes applied to banded obsidian.

Peanut obsidian - a gray to greenish gray perlite that contains red or brownish red stellate spherulites, which consist of radiating fibers of hematite-stained feldspar within shells of chalcedony; the overall appearance -- both color- and size-wise -- roughly resembles a mass of peanuts. A noteworthy occurrence is near Alamos, State of Sonora, Mexico. Pearlylite - name sometimes given in the marketplace to obsidian in jewelry. Pitchstone - obsidian that has a pitchlike luster. This rock apprently represents an intermediate phase in devitrification of the precursor obsidian per se; among other things, it contains more water and is less brittle than typical obsidians. Plum obsidian - obsidian with a plum (purplish) color. Pumpkin obsidian - obsidian with a pumpkin-orange color. Porphyritic obsidian - obsidian containing sporadic phenocrysts.

Rainbow obsidian (also rainbow sheen obsidian and iris obsidian) - obsidian that exhibits a multicolored iridescence, apparently because of the presence of inclusions -- e.g., that from Glass Buttes, Lake County, Oregon and that, called Mexican 'Mayan' from the state of Jalisco, Mexico (see Koivula, Kammerling and Fritsch, 1993). See Fig. C. Royal agate - misnomer sometimes applied to mottled obsidian. Rhyolite glass - name apparently given by someone who knew that most obsidian is chemically like rhyolite but did not know that rhyolite is an aphanitic rock consisting only in part of glass. Royal blue obsidian - a bluish obsidian from northeastern California. Schiller obsidian - obsidian exhibiting a schiller effect. Sheen obsidian - obsidian that exhibits a sheen when rotated in reflected light; many marketers distinguish diverse "sheen obsidians" by directing attention to the color of their sheen -- e.g., golden ... , silver ..., and even rainbow sheen obsidian.

Snowflake obsidian (also spherulitic obsidian and flowering obsidian) - dark gray to nearly black obsidian with inclusions of white, gray or rarely red spherulites, which in many specimens have been identified as cristobalite (a high-temperature polymorph of silica). Fine examples of this obsidian occur near Milford, Beaver County, Utah. See Fig. B. "Star" obsidian (also "starred Apache tear obsidian") - misnomer -- in that no asterism is apparent -- applied in the marketplace to gemstones cut from aventurescent (hematite- and ilmenite-bearing) obsidian, most of which occur as Apache tears (see Koivula & Tannous, 2003). Velvet obsidian - obsidian with a velvetlike appeareance. Volcanic glass - geological designation sometimes applied to obsidian used as a gemrock. Xaga - name used for obsidian by some native Americans from California. The Pomo Indians are said to have distinguished between hard obsidian, which they called dupa xaga and used for razors etc. and less hard obsidian, which they called bati xaga.

USES: Obsidian is frequently carved for jewelry -- especially earrings, bracelets and pendants -- and into diverse ornaments, such as busts and animals, made especially for the souvenir trade, and rod and needlelike pieces of obsidian have been incorporated into wind chimes. Especially in ancient times, because it breaks to give sharp edges, obsidian was sought and used for cutting tools, weapons and ceremonial points such as spearheads; indeed, even today replica arrow heads are made from obsidian for use as pendants or incorporation in decorative items, including jewelry.

In addition, obsidian has been carved into diverse vessels such as chalices and vases, and incorporated into statues (etc.) as pupils of eyes (Weiner, 1983), and Pough (ms) reports that "in Mexico, Eduardo Obe's, of Morellos creates giant sculptures for art galleries and tequila cups [from obsidian] for his opening night patrons." A rather ancient "tangential" use, which relates to visual "art," seems to warrant inclusion here: "Obsidian flakes with retouched points were used as tattooing tools [ that were involved in social, ritual and/or medical practices ~] 3000 BP at the Nanggu [archaeological] site [in the Solomon Islands]" (Choi, 2016; Kononenko, Torrence & Sheppard, 2916). OCCURRENCES: As masses of rapidly cooled, silica-rich magma associated with relatively recent (geologically speaking) volcanism. MacDonald, Smith and Thomas (1992) list lava flows, extrusive domes and blocks in pyroclastic deposits as loci.

NOTEWORTHY LOCALITIES: Parts of Italy; the Caucasus Mountains of the former U.S.S.R; and areas in Mesoamerica -- e.g., the vicinity of Real Del Monte, State of Hidalgo, Mexico (especially for sheen obsidian) -- have been major sources. Also, several localities in the western United States are well-known for one or another kind of obsidian that has been used as a gemrock -- e.g., Mariposa and Pinal counties, Arizona; Glass Mountain, south of Lava Beds National Monument, Siskiyou County and near Davis Creek in adjacent Modoc County, northern California (especially for rainbow obsidian, but also for other varieties -- see Mitchell, 1987); near Milford, Beaver County Utah (especially for rainbow obsidian, but also for several other varieties, especially Apache tears); Glass Buttes, in northeastern Lake County, Oregon (photographs of fire, gold sheen, mahogany and rainbow obsidian from this locality are given in Pough, 1996); Millard County, Utah (especially snowflake obsidian); and Yellowstone National Park, Wyoming. See also Weiner (1983).

REMARKS: The term obsidian is said to have resulted from a printer's error involving the Latin word Obsianus, which was meant to allude to Obsius, who, according to Pliny, the Elder (Book xxxvi), discovered this rock in Ethiopia (formerly Abyssinia). Obsidian is said to have been artificially decolorized -- see Nevada diamond under OTHER NAMES subheading. Obsidian that exhibits flow lines has been cut into thin slices and cemented on to, for example, synthetic spinel, to made doublets with the flow lines of the slices oriented in selected directions. Some rainbow obsidian has been fashioned for pendants so it exhibits heart-shaped reflections; these pieces are produced by cutting a groove from near the edge to about the center on the flat side of a suitably shaped piece such as a relatively thick cabochon. Obsidian has been used as mirrors, for example, in telescopes. However, as noted by Pliny the Elder (ibid.), these so-called mirrors reflect silhouettes (umbras) rather than images like those seen in what most people call mirrors. Pliny also reports that Caesar Augustus was the emperor who so-to-speak commissioned carving of the obsidian elephants in the Temple of Concord.

The designation Apache tears is apparently based on American Indian lore: These masses were said to have formed when Apache warriors leaped from a cliff to their death rather than being captured by an enemy. Frederick Pough (ms) notes that "In Superior, Arizona, a cut along a cliff reveals thousands of staring black 'eyes' [the Apache tears], peeping out from gray flaky lids .... [And, he also notes that being] full of water, [these] marble-size nodules swell to baseball dimensions on being heated with this behavior giving expanded perlite,[which has] a role in building insulation." -- i.e., perlite is used as a light weight aggregate in such things as sound-absorbing tiles. The Aztec term iztli was so-to-speak surnamed teotetl, which has been translated as divine stone, apparently in reference to its many uses -- Aztecs fashioned both tools and items for adornment from obsidian. An especially noteworthy piece is the famous "monkey bowl," said to have been used in rituals involving human sacrifice, at one time exhibited in the National Museum of Anthropology in Mexico City, Mexico.

Items fashioned from basaltic composition glass, properly called sideromelane or tachylyte, are frequently called obsidian, especially in the marketplace. Petrologically, different origins of their parent magmas and their different compositions distinguish basaltic glasses from obsidians. So far as uses of basaltic glass as a gemrock, pieces made from them are extremely rare as compared to those made of obsidian. Two things, in particular, appear to be responsible for this difference: A.basaltic glasses are uncommon whereas obsidian is relatively common; and B.baslatic glasses, unlike obsidians, tend to dissolve readily in acids. Webster (1994, p.290), however, does record cabochons fashioned from different colored basaltic glasses from the vicinity of Flinders River, northern Queensland, Australia. SIMULANTS: ***Emerald obsidianite (also called Mount St. Helens obsidian) - a man-made glass that has been made to be sold as a souvenirs of the May 1980 eruption of Mount St. Helens, Washington. - [Appearance suffices.].

Glass portions of impactites -- [for pieces in, for example, small carvings, some of these could be distinguished on the basis of chemical analyses or even microscopic means; others might be nearly impossible to distinguish from obsidian unless their occurrence is known]. ***Glassy slag - [for some -- determination of total composition is necessary, and this requires use of relatively sophisticated equipment]. ***Helenite - glass said to have been produced by melting volcanic ash and/or dust erupted by Mount St. Helens, Oregon in 1980, that has been molded into, for example, subspherical and egg-shaped masses and sold for use as paper weights and curios. The purported origin of the material used to make at least some of these glass pieces has, however, been questioned by Nassau (1986). [For all of these I have seen appearance suffices. By the way, Nassau's conclusions about the origin of the precursor material for this simulant are based on chemical analyses.]. ***Man-made glasses of diverse compositions, including glassy slags from some smelters.

Some bluish slags resemble hyalite opal -- e.g., some of those from the old iron furnace in the area now set aside as part of the Layette State Park, on Big Bay de Noc, Delta County, Michigan, where they occur as cobbles and pebbles on Slag Beach (I collected some of these for a lapidary friend in the early 1970s.). Several years later, I read that an apparently similar slag from Sweden is being fashioned into, for example, attractive cabochons (Johnson and Koivula, 1999) - [Some, but not all, of these are not distinguishable as manufactured without laboratory investigation.] Along this line, most of the glass usually referred to as "Beach Glass" is Man-made glass. In any case, the North American Sea Glass Association has recently held a annual contest that makes awards for to collectors for some of the diversely colored pieces of glass that have been given their current characteristics -- e.g., smooth edges -- as a result of erosion they have undergone within oceans, the Great Lakes, etc. . Tektites -- [same remarks as those given for "Glass portions of impactdites]. See TEKTITE entry.

Note: This listing raises another topic of at least tangential interest: A number of green gemstones, many of which have been faceted, have been marketed as green obsidian. Johnson, Reinitz and Owens (in Moses, Reinitz and McClure, 1998), however, report that "We cannot recall seeing an example of transparent 'green obsidian' that has ever been proved to be a natural obsidian." REFERENCES: Montgomery, 1981; Pough, ms in preparation, 1997; VanLandingham, 1962, pt.2; Weiner, 1983. Obsidian OBSIDIAN Obsidian is a natural glass of volcanic origin that is formed by the rapid cooling of viscous lava. Obsidian is extremely rich in silica, low in water, and has a chemical composition similar to rhyolite. Obsidian has a glassy luster and is slightly harder than window glass. Though obsidian is typically jet-black in color, the presence of hematite (iron oxide) produces red and brown varieties, and the inclusion of tiny gas bubbles may create a golden sheen.

Other types with dark bands or mottling in gray, green, or yellow are also known. Obsidian generally contains less than 1 percent water by weight. Under high pressure at depth, rhyolitic lavas may contain up to 10 percent water, which helps to keep them fluid even at a low temperature. Eruption to the surface, where pressure is low, permits rapid escape of water and increases the viscosity of the melt. Increased viscosity impedes crystallization, and the lava solidifies as a glass. Different obsidians are composed of a variety of crystalline materials. Their abundant, closely spaced crystallites (microscopic embryonic crystal growths) are so numerous that the glass is opaque except on thin edges. Many samples of obsidian contain spherical clusters of radially arranged, needlelike crystals called spherulites. Microlites (tiny polarizing crystals) of feldspar and phenocrysts of quartz may also be present. Obsidian was used by American Indians and many other primitive peoples for weapons, implements, tools, and ornaments and by the ancient Aztecs and Greeks for mirrors.

Because of its conchoidal fracture (smooth curved surfaces and sharp edges), the sharpest artifacts were fashioned from obsidian; some of these, mostly arrowheads, have been dated by means of the hydration rinds that form on their exposed surfaces through time. Obsidian in attractive and its variegated colors make it useful as a semiprecious stone. How to Find Obsidian Rocks | eHow How to Find Obsidian Rocks Tom Brakefield/Stockbyte/Getty Images Obsidian, or volcanic glass, is not only a beautiful ornamental rock, it was once used by Native Americans to craft arrowheads and cutting tools because of its strength and sharp edges. There are many localities that contain obsidian deposits in the United States, and knowing how and where it originally formed will provide significant information on locating these outcrops for collecting. It is also important to understand the variations in color, which will assist in identifying deposits of volcanic glass. Things You'll Need Geologic map Rock identification guide Learn about obsidian's origins and how it is formed. Volcanic glass is an igneous rock created when silica-rich lava is extruded directly into water. The instant cooling does not allow time for crystals to grow as in other igneous rocks.

This creates a glassy characteristic because the minerals are too small to see individual crystals, as in granite or other igneous rocks. Individual crystals can be seen plainly in granite and other igneous rocks. Use field guides to gain a general knowledge of what obsidian looks like, especially its color and mineral variations. Despite its silicate composition, which typically produces white or clear minerals, obsidian is dark because the minerals remained mixed like a soup. Certain mineral inclusions can give obsidian different colors. For instance, iron and magnesium can give the rock a green appearance, while hematite will produce reddish-brown hues. Obtain a geologic map of obsidian localities. The mountainous regions of the western United States have a lengthy geologic history of volcanism which created a mass of obsidian deposits. Mines, claims and other known locations are found in abundance in Arizona, California, Colorado, Oregon and Utah, among others. To locate these regions, contact the U.S. Geological Survey (USGS) or the Association of American State Geologists (AASG) for maps and information. A list of known mining sites can be obtained from mindat.org, which can provide a basis for searching the right areas.

Get permission to go rock collecting. No matter where the deposits of obsidian are located, always check with the property owner before attempting to collect on the land. Rock collecting in general is prohibited in many national and state parks, as well as mines or other commercially owned areas. Collecting from road outcrops and cliff faces is not only illegal, but also very dangerous. Be sure to wear proper gear and always get permission to be on that site.

Question: Which glassy, dark volcanic rock was used by some to make edged tools and arrowheads?
Answer: OBSIDIAN