Spinel Inclusions • An Exercise in Aesthetics

To the jeweler, spinel is famous for its vivid colors.  But for the gemologist, this gem is unlike any other.  Its extreme hardness allows a fine polish.  Couple this with single refraction, which eliminates the image blurring found in most other gems, and a varied landscape of inclusion subjects, and the result is an unparalleled canvas of delight for the photomicrographic artist. 

Nothing is beautiful, only man: on this piece of naïveté rests all aesthetics, it is the first truth of aesthetics.  Let us immediately add its second: nothing is ugly but degenerate man - the domain of aesthetic judgment is therewith defined.  Friedrich Nietzsche

aes·thet·icsnoun
Aesthetics, or esthetics (/ɛsˈθɛtɪks, iːs-, æs-/) is a branch of philosophy that deals with the nature of art, beauty and taste and with the creation or appreciation of beauty.  In its more technical epistemological perspective, it is defined as the study of subjective and sensori-emotional values, or sometimes called judgments of sentiment and taste. 

Introduction to spinel inclusions

The appreciation of gemstones is, at its heart, an exercise in aesthetic “subjective” judgment.  Yet, both gem dealers and gemologists reach for “objective tools” (instruments) to describe these inherently subjective phenomena. 

Gemologists employ ever-more complex instruments in an attempt to characterize the gifts of nature they are so blessed to handle.  Virtually all of these deliver information in an entirely abstract fashion, with a series of numbers or graphs that have no relationship to the emotions people feel when gazing at a beautiful gem. 

The one instrument that does not neuter the senses is the microscope.  When one views a gemstone’s interior under magnification, the “read out” is replaced by a sense of awe and wonder.  In a lifetime spent being assaulted by time control, where life is broken into ever smaller digital time bits, suddenly you plunge into a world of frozen time, one that affords vistas of millions or even billions of years.  You witness the primordial forces that birthed our planet and, indeed, our entire universe.  Entering this dimension is humbling; comparing the 5000 years of human “civilization” with the vastness of this “inner space” is a psyche-shattering experience. 

In what follows, we will introduce readers to this magical realm, as found in the gem spinel.  Like a frame around a painting, science is used where appropriate to enhance the subject, but it is our hope that this will only increase your enjoyment of these aesthetic masterpieces of Mother Nature. 

Geology of Spinel Deposits

The majority of gem spinel deposits around the world are of metamorphic origin, where spinel occurs associated with marble.  These include:

In addition, there are alluvial gem spinel deposits where the nature of the original host rocks is unknown.  These include:

Madagascar, Mozambique, Nigeria, Sri Lanka, Tanzania (Tunduru, Umba)

Deposits in East Africa and Sri Lanka generally formed 730–550 million years ago, associated with the Pan-African orogeny, while those associated with the Himalayan orogenic event formed 5–25 million years ago (Giuliani & Fallick et al. ( 2017). 

Inclusion Types

Primary cavities

When gemology students are taught about spinel, one of the first things they are told to look for are octahedral crystals.  These echo the external form of spinel crystals and look like little bipyramid shapes inside the spinel.  Spinel fingerprints were first described by B.W. Anderson in 1942 (Anderson, 1947).  In spinel, these negative crystals are typically filled with white (dolomite), yellow (phlogopite) or black (hercynite) matter. 

One of the authors (JIK) coined the term “philoxenism” to describe a host’s propensity to fill its cavities with a foreign mineral (Gübelin & Koivula, 2005).  This certainly apples to spinel, for this crimson lady is the master dominatrix in this regard, forcing its external octahedral shape on a variety of guests. 

Secondary Cavities

In contrast to their corundum cousins, while they are known, fluid inclusions are relatively rare in spinel. 

Solids

Spinel is host to a rich variety of solids.  These can be placed into the following categories according to a scheme first developed by Gübelin (1973):

Protogenetic: Solids that formed before the host

Syngenetic: Solids that formed simultaneously with the host

Epigenetic: Solids that formed after the host.  These form through a process of exsolution. 

Secondary (Exsolved) Solids

Similar to corundum, spinels sometimes contain exsolved rutile.  In spinel, these form along the edges of octahedron faces.  When present in enough quantity, star spinels can be cut.  Other exsolved solids in spinel include titanite (sphene) and högbomite. 

A spinel octahedron, frozen in time; Sri Lanka.  Photo: E. Billie Hughes; click on the image for a larger photo

Left: Like a piece of meat on a satay stick, an apatite crystal rod pierces a spinel crystal in spinel; Field of view: ~2 mm.  Right: Small transparent crystals form clusters in a Vietnamese spinel.  These tabular crystals are transparent and doubly refractive, but their host's morphology has been forced upon them, a characteristic John Koivula has dubbed "philoxenism."  Dark field illumination.  Photos: E. Billie Hughes; click on an image for a larger photo. 

Left: Sulfur-rich multi-phase fluid inclusions in a natural pink spinel from Pein Pyit, Mogok, Burma. Right: The same sulfur-rich fluid inclusion after cutting down to expose it at the surface reveals that the orange material is a fluid that has leaked out, revealing it to most likely be a hydrocarbon (like petroleum oil).  Photos: Richard W. Hughes; click on an image for a larger photo. 

Left: This “belly button” apatite crystal is a classic inclusion in spinel.  The black belly button is a crystal of either ilmenite or graphite on the surface of the rounded apatite crystal.  These are common features of spinels from both Myanmar and Sri Lanka. Right: Transparent birefringent crystals of apatite in spinel a spinel from Myanmar with "belly button" spots under crossed polars.  Photos: E. Billie Hughes; click on an image for a larger photo. 

Left: Reflecting light, skeletal crystals of syngenetic titanite (sphene) cause aventurescence in a host spinel from Sri Lanka.  Field of view: 2.0 mm. Right: Crystallographically oriented acicular inclusions of ilmenite and rutile result in 6-rayed asterism in their host spinel from Sri Lanka.  Field of view: 3.4 mm.  Photos: Nathan Renfro; click on an image for a larger photo. 

Left: A prominent, intricate fingerprint pattern consisting of interconnected communication tubes extending from a mineral inclusion nucleus serves to confirm that this Sri Lankan spinel is natural.  The form taken by the tubes reveals the isometric symmetry of the underlying spinel.  Magnification: 5XRight: Xenomorphic carbonate (probably dolomite) filling an octahedral negative crystal with oriented needles at the corners.  The needles might be either mineral matter or acicular voids.  Spinel host from Burma (Myanmar).  Magnification: 200X. Photos: John I. Koivula; click on an image for a larger photo. 

Left: Swirls of exsolved particles light up the interior of this red spinel from Tanzania’s Mahenge region. Right: When fissures in spinel heal, they often do so in a distinctive pattern of octahedral negative crystals which align in rows according to the underlying atomic symmetry, as show in this spinel from Tanzania’s Mahenge district.  Photos: Richard W. Hughes; click on an image for a larger photo. 

Left: This large void in a Sri Lankan spinel contains a liquid and a gas bubble as well as opaque black graphite flakes.  Field of view: 5.8 mm.  Right: Comparing this image to the previous one clearly shows that the contents in the 3-phase inclusion are free to move about in the void.  Field of view: 5.8 mm.  Photos: John I. Koivula; click on an image for a larger photo. 

Left: Color-causing sheets of hematite are oriented along octahedral planes in this spinel from Sri Lanka.  Field of view: 5.0 mm.  Photo: John I. Koivula; Right: This light gray spinel from Mogok, Myanmar, contains a vibrant blue inclusion of lazurite.  Field of view: 1.59 mm.  Photo: Nathan Renfro; click on an image for a larger photo. 

Left: This rounded and corroded protogenetic pyrite inclusion in a Sri Lankan spinel shows a deeply pitted surface.  Field of view: 3.9 mm. Right: An iridescent booklet of translucent white muscovite mica resides in the interior of a mauve spinel from Sri Lanka.  Field of view: 3.5 mm.  Photos: John I. Koivula; click on an image for a larger photo. 

Radiating fibers create an otherworldly star burst in this Mogok spinel; Myanmar.  Photo: Wimon Manorotkul; click on the image for a larger photo. 

This rounded protogenetic zircon inclusion in a Sri Lankan spinel shows colorful strain in polarized light.  Field of View: 2.1 mm.  Photo: John I. Koivula; click on the image for a larger photo. 

Mirror-smooth hexagonal graphite crystals decorate the interior of a light bluish gray spinel; Sri Lanka.  Field of View: 2.1 mm.  Photo: John I. Koivula; click on the image for a larger photo. 

Syngenetic prisms of fluorapatite hitch a ride in a blue spinel from Sri Lanka.  Photo: Nathan Renfro; click on the image for a larger photo. 

Summary of spinel inclusions

As any good artist knows, it is a fallacy to think that two people of normal eyesight gazing at a given object are necessarily seeing the same thing.  It is the interpretation of the image that enables the artist to see a significant and exciting pattern of shapes, shadows and colours, where his friend may see nothing of any interest whatsoever.  B.W. Anderson, 1966, "Gemmology on a Shoestring"

To the jeweler, spinel is famous for its vivid colors.  But for the gemologist, this gem is unlike any other.  Its extreme hardness allows a fine polish.  Couple this with single refraction, which eliminates image blurring, and a varied landscape of inclusion subjects, and the result is an unparalleled canvas of delight for the photomicrographic artist. 

About the authors

E. Billie Hughes, the youngest member of the Lotus Gemology fold, visited her first gem mine (in Thailand) at age two and by age four had visited three major sapphire localities in Montana.  A 2011 graduate of UCLA (B.A., Political Science), she qualified as a Fellow of the Gemmological Association of Great Britain (FGA) in 2013.  Billie's photographic work has been published in Terra Spinel, the Wall Street Journal, Ruby & Sapphire: A Collector's Guide and Ruby & Sapphire: A Gemologist's Guide.  To date, she has visited scores of countries for research on gems, including the US, Thailand, Cambodia, Vietnam, Myanmar, Sri Lanka, India, Rwanda, Malawi, Tanzania, Mozambique, Madagascar, Kenya and Greenland, and has delivered lectures in Thailand, China, Sri Lanka and the US.  Her articles, gemological images and photomicrographs have appeared in Gems & Gemology, The Gemguide, The Journal of the Gemmological Association of Hong Kong, and InColor magazine.  She is a talented photomicrographer and a three-time winner of the Gem-A's annual photographic competition. 

John I. Koivula, B.A., B.Sc.,  G.G., F.G.A., Fellow Royal Microscopical Society is the co-author of the magnificent Photoatlas of Inclusions in Gemstones, Vols.  1–3 and the author of the MicroWorld of Diamonds, along with several other books and numerous articles.  He is currently Analytical Microscopist at the Gemological Institute of America and is the world's foremost gem photomicrographer and inclusionist.  John’s images have graced the covers and contents of numerous books and journals.  In addition, he won 1st Place and others in Nikon’s Small World photomicrographic competitions.  Koivula is an honorary life member of both the Finnish Gemmological Society and the Gemmological Association of Great Britain, and was named as one of the 64 most influential people of the 20th century in the jewelry industry by Jewelers' Circular Keystone magazine and one of the 50 most important figures that have shaped the history of gems since antiquity by the Association Française de Gemmologie (AFG).  John was bestowed The Richard T. Liddicoat Award from GIA in 2009.  He also has been awarded the Robert M. Shipley Award by the American Gem Society, the Scholarship Foundation Award by the American Federation and California Federation of Mineralogical Societies, the Antonio C. Bonanno Award for excellence in gemology by the Accredited Gemologists Association, and Koivula was the first recipient of the Richard T. Liddicoat Journalism Award from the American Gem Society.  John was also the technical and scientific advisor to the famous MacGyver television series from 1986–1993.  Many of his books can be seen at www.microworldofgems.com, and are available from the GIA and Gem-A bookstores. 

Wimon Manorotkul has been involved with gems and gemology since 1979, as a lab gemologist, instructor and photographer.  She is an Accredited Gemologist from Bangkok's Asian Institute of Gemological Sciences and for many years directed their lab.  Wimon also qualified as a Fellow (with honors) of the Gemmological Association of Great Britain.  A skilled gem photographer, her images have been featured in books and magazines around the world, particularly Ruby & Sapphire: A Collector's Guide and Ruby & Sapphire: A Gemologist's Guide.  Wimon not only photographs gems, jewelry and mineral specimens, but is also an expert photomicrographer.  In 2013, she founded Lotus Gemology with her husband, Richard Hughes, and daughter, E. Billie Hughes. 

Nathan Renfro’s interest in minerals was sparked in his late teens when he caught a glimpse of his grandfather’s rock collection.  From that point, there was no turning back.  In 2006, he completed his undergraduate studies in geology and then enrolled in GIA’s Graduate Gemologist (GG) program at the Carlsbad campus as a recipient of the William Goldberg Diamond Corporation scholarship.  After earning the GIA GG diploma, Nathan was hired by GIA as a diamond grader.  In 2008 he joined the Gem Identification department, in Carlsbad California, where he currently holds the position of Manager/Microscopist.  Nathan has authored and co-authored dozens of articles on gemology, and has lectured to gem and mineral groups throughout the United States.  His primary areas of interest are inclusion identification, photomicrography, lapidary arts and the defect chemistry of corundum. 

Notes

An abridged version of this article was published in InColor (2019, No.  43, Summer, pp.  66–73).