 Faceted black spinel 26 x 20 mm Photo: E. Skalwold. |
and other Black Gems of Thailand Dr. Seriwat Saminpanya of Srinakharinwirot University, Thailand and Dr. Frederick Lin Sutherland of the University of Western Sidney, Australia characterize in their recent paper the properties of three black opaque gem minerals little known outside of Thailand : Black Opaque Gem Minerals Associated with Corundum in the Alluvial Deposits of Thailand. Australian Gemmologist, Vol. 23, No. 6, pp. 242-253. |
| When they consider a black gem most people think of "black onyx," a chalcedony which has been dyed to achieve a uniform black color. Relatively recently faceted black diamonds and black spinel have made an entrance into the world of designer goldsmiths, but for the most part, dyed chalcedony remains the dominate black material seen. Spinel, pyroxene and magnetite are three black opaque gem materials found in the same Thai alluvial deposits as corundum. Though these untreated by-products of sapphire mining may be higher luster and harder than chalcedony, making them superior choices for fashioning into beautiful black faceted gems for jewelry design, they are under-utilized due to name confusion and lack of information about them (Saminpanya, 2008). While living in Thailand, I found an abundance of these gems in the markets both as faceted stones and as finished jewelry pieces, including faceted beads. They had a very high luster and very sharp facets, unparalleled by chalcedony, though much was sold as "black onyx." Most of what is seen in the market is spinel or pyroxene which can not be easily distinguished from each other by the unaided eye (Saminpanya, 2008). In their extensive 12 page paper, the authors explore the properties of these gem materials, along with their geological environment, seeking to clarify misconceptions surrounding them and to determine their suitability as a viable gem material. | |
 Gem gravel being washed into sorter. Photo courtesy of Dr. Seriwat Saminpanya. | |
 Spinel octahedral crystals. Photo courtesy of Dr. Seriwat Saminpanya. |
Black spinel is well known in Thailand as nin, which means "black gem." Locally it is also known as nin-ta-go or nin-tan, both of which approximately translate as " looks solid without surface striation." It is considered an indicator mineral by miners seeking corundum bearing deposits. These miners refer to spinel as "blackjack" when they come across it in the alluvial gravel (Saminpanya, 2008). Black pyroxene and magnetite are both found in the same deposits as the corundum and spinel. Pyroxene is locally known as nin-sian (having striations parallel to the cleavage direction). Magnetite is called nin-tid-lek, loosely translated as "magnetic." Both black spinel and pyroxene are sold by the misnomers black sapphire and onyx (Saminpanya, 2008). A summary of the gemological properties of all three minerals is summarized in a chart in the paper (black spinel: spinel-hercynite; black pyroxene: augite; magnetite: magnetite-ulvöspinel). |
 with surface cleavage striations. Photo courtesy of Dr. Seriwat Saminpanya. |
 showing strong magnetism. Photo courtesy of Dr. Seriwat Saminpanya. |
 black opaque gem mineral by-products of sapphire mining. Photo courtesy of Dr. Seriwat Saminpanya. | |
Publications by Dr. Seriwat Saminpanya:
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