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The Splendour of Spinel, part 1: A Cousin to Corundum

Species

Spinel

Chemical Formula

MgAl2O4

Mohs Hardness

7.5 - 8

Cleavage

None

Luster

Vitreous to Subadamantine

Refractive Index

1.719 - 1.920

Birefringence

None

Dispersion

0.020

Pleochroism

None

Specific Gravity

3.58 - 3.98

 

Although lesser known than the likes of ruby and sapphire, spinel is regarded as being among the more important gemstones of human history. It remains a desirable participant in gemstone jewellery to this day, and it has also recently taken its place alongside peridot as one of the official contemporary western birthstones for the month of August. Spinel is the mineral form of magnesium aluminum oxide, a close chemical cousin to corundum, the mineral form of aluminum oxide and the parent species to which ruby and sapphire belong. The name for this mineral is derived from the Latin word “spinella”, meaning “spine”, in reference to the pointed and spiny looking octahedral shape in which spinel crystals often form. Most famous for the red colouration they sometimes display, spinel gemstones share a number of superficial characteristics with closely related ruby and sapphire, often being confused for gem corundum in the ancient world due to a lack of knowledge in the advanced physical chemistry which was later used to definitively distinguish the two minerals from one another. 

 

An octahedral red spinel crystal perched on a matrix of white calcite, excavated in the Mogok mining district of Myanmar’s Mandalay Region; Image: Rob Lavinsky/ Mindat

 

In many ways, spinel shares characteristics with closely related corundum gemstones. The lustre of spinel can be sub-adamantine, approaching the lustre of diamond but not replicating it, and in some cases spinel’s refractive index is quite close to that of corundum as well. Similar to corundum, spinel is a mineral with very high hardness making it suitable for use in rings, although it is not of equal hardness to corundum. These similarities, in combination with the various hues spinel may exhibit, allow for many cases when designing bespoke coloured gemstone jewellery where spinel gems may be used interchangeably with sapphires and rubies. Unlike corundum, spinel crystals with high degrees of transparency are a more frequent occurrence in nature than highly transparent corundum crystals, and consequently spinel gemstones typically exhibit greater clarity than most unenhanced rubies and sapphires; their desirable clarity and lower price point frequently make spinel gems budget friendly options for alternatives to ruby and sapphire, particularly if one is designing an engagement ring and wishes to use a less conventional stone which is of good clarity and nearly equal in beauty to corundum.

 

A purple spinel gemstone, which in many ways resembles a purple sapphire; Image: SkyJems

 

An important mineral, spinel has given its name to the larger “spinel group”. Spinel is the titular member of the larger mineral group and the smaller aluminospinel family, which contains crystallographically cubic minerals with the formula XAl2O4, where the X site may be occupied by a number of divalent metals including magnesium (Mg, i.e. “spinel”), zinc (Zn, i.e. “ghanite”), ferrous iron (Fe2+, i.e. “Hercynite”), manganese (Mn, i.e. “galaxite”), or both magnesium and ferrous iron (Mg and Fe2+, i.e. “pleonaste”); other spinel families also exist, such as ferrospinels (XFe2O), chrome spinels (XCr2O), and vanadium spinels (XV2O). Crystals of spinel typically form in a matrix of soft carbonates, such as marble and dolomite, or may form within harder igneous rocks. Spinel crystals may also be encountered as alluvial deposits due to weathering of the soft matrix minerals in which the crystals often grow. From these sources, soft matrix extraction and alluvial sifting are some of the most common methods for obtaining rough spinel. Very frequently spinel will occur in close proximity to gem grade corundum; in some cases the two minerals will even form on top of one another. This co-occurrence has been a significant factor in the historical misidentification of spinel as ruby and/or sapphire, and it can still prove bothersome during field expeditions when adequate testing equipment is inaccessible.

 

A red spinel and diamond ring crafted in 18 karat yellow and white gold. The red spinel is virtually indistinguishable from a ruby to the untrained eye; Image: SkyJems

 

Spinel is found all over the globe but deposits bearing notable quantities of gem grade material are few in number. For large parts of history, Southern Asia and Southeast Asia were the primary sources for this gemstone, with the gem bearing gravels of Ratnapura in Ceylon (a.k.a. Sri Lanka) and the gem mines of Mogok in Burma (a.k.a. Myanmar) being among the greatest producers of spinel gems. During the middle ages, high quality spinel crystals were discovered in the Badhakshan region of what is now northern Afghanistan and eastern Tajikistan. These mines did not maintain their operations, but there have since been efforts to unearth more spinel in the areas which surround the site of the original Tajik mines. Other important sources for spinel which have become active in recent history include the Yen Bai Province of Vietnam, and the Morogoro Region of Tanzania. To a lesser extent, spinel gemstones are also mined in Kenya, China, Pakistan, Cambodia, Nepal, Australia, Nigeria, and Madagascar, although production from some of these sources is limited.

 

Cobalt blue spinel crystals on a matrix of white albite feldspar from Lục Yên District, Yên Bái Province, Vietnam; Image: Duong Hong Duong/ Mindat

 

In part due to its importance as a member of the gemological and mineralogical lexicon, spinel has left its mark on crystallographic vocabulary: a particular kind of crystal growth habit is now referred to as “spinel law twinning”. When a crystal “twins”, it grows in conjunction with a second crystal which shares at least one of its crystallographic axes. In the case of spinel law twinning, two octahedral crystals grow with a shared triad axis about which one crystal is rotated 180 degrees from the first. This type of crystal twinning is not exclusive to spinel and can be seen in numerous other members of the cubic crystal system such as fluorite, pyrite, and galena, but spinel law twins are most commonly encountered in the gemstone trade when handling rough diamonds; the term “macle” is used to refer to tabular diamond crystals which exhibit spinel law twinning, but it is also sometimes applied to twinned spinel crystals. What sets spinel twins apart from those formed from other materials is the twinning shape sometimes called a “Star of David” twin, named so because of a striking similarity between the shape of these crystals and the Star of David symbol that originates from the Jewish faith; such spinel twins are composed of two tabular octahedrons which have grown parallel to one another but pointed in opposite directions, giving the impression of a six pointed star.

 

A more typical presentation of spinel law twinning in a red spinel crystal from Mogok, Myanmar; Image: Rob Lavinsky/ Mindat
A red spinel crystal that has grown as a flattened “macle” twin on a matrix of white calcite from Mogok, Myanmar; Image: Albert Russ/ Mindat
A “macle” diamond crystal from the Northern Cape of South Africa; Image: Rob Lavinsky/ Mindat
A red spinel “Star of David” twin, from Mogok, Myanmar; Image: Rob Lavinsky/ Mindat

 

In the next part of this series, learn about the different forms which spinel gems may take, and the different ways in which humans may influence their beauty.

 

© Yaĝé Enigmus


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