Feature Gems & Gemology, Winter 1996, Volume 32, No. 4

Some Gemological Challenges in Identifying Black Opaque Gem Materials

Mary L. Johnson, Shane F, McClure, and Dino G. DeGhionno

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Among the most difficult gems to identify are those that are black and opaque (or nearly so). In general, any gem material can be opaque because of inclusions, any black opaque material can be fashioned, and any porous material can be dyed. Thus, to identify a black opaque material, every possible mineral, and many rocks and manufactured substances, must be considered. Microscopic appearance, refractive index, specific gravity, and other properties (such as magnetism or radioactivity) provide useful clues, but in most cases advanced identification techniques (X-ray diffraction, EDXRF spectroscopy) are necessary, and even these may not be conclusive. Black opaque pyroxenes, amphiboles, and spinel-group minerals are especially challenging to identify.