Some Gemological Challenges in Identifying Black Opaque Gem
By Mary L.Johnson, Shane F, McClure, and Dino G. DeGhionno Among the most difficult gems to identi- ne of the most challenging problems in gemology is that fy are those that are black and opaque (or of determining the identity of a blaclz opaque gem inate- nearly so). In general, any gem material 0rial-hereafter called a "blaclz opaque." Such materials are can be opaque because of inclusions, any a mainstay of the jewelry business, primarily as side stones, dec- black opaque material can be fashioned, orative elements in multi-stone mosaics, and in men's jewelry. and any porous material can be dyed. Treated blaclz chalcedony ("black onyx"), black jade, and Thus, to identify a black opaque materi- al, every possible mineral, and many hematite traditionally have been the black opaques in greatest rocks and manufactured substances, demand. As a variety of quartz, "black onyx" is probably the must be considered. Microscopic appear- most familiar durable black opaque to lapidaries and gem cut- ance, refractive index, specific gravity, ters. Today, it is a popular medium for artistic carvings, many of and other properties (such as magnetism which have been incorporated into fine jewelry (figure 1). or radioactivity) provide useful clues, but In recent years, various materials have been misrepresent- in most cases advanced identification ed as "black onyx" or "black jade" to meet the trade's need for techniques (X-raydiffraction, EDXRF calibrated goods in high-volume markets. Members of the spectroscopy) are necessary, and even trade, in turn, have been sending samples to identification lab- these may not be conclusive.
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