gemstone
Black Onyx
Chalcedony (SiO2)
Hardness of 6.5 to 7 on the Mohs scale, color is solid black, luster ranges from vitreous to waxy, crystal structure is microcrystalline (cryptocrystalline quartz), cleavage is none, and specific gravity is approximately 2.60 to 2.65.
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Physical properties
Hardness of 6.5 to 7 on the Mohs scale, color is solid black, luster ranges from vitreous to waxy, crystal structure is microcrystalline (cryptocrystalline quartz), cleavage is none, and specific gravity is approximately 2.60 to 2.65.
Formation & geological history
Forms in the gas cavities of volcanic lava, through the deposition of silica from silicious thermal waters in various geological environments.
Uses & applications
Extensively used in fine jewelry, signet rings, carvings, cameos, and ornamental beads.
Geological facts
Onyx has been prized since ancient Egyptian, Greek, and Roman times for intaglio carvings and protective amulets. Most commercial black onyx today is actually common grey or brown chalcedony that has been artificially treated and dyed black to achieve uniform color.
Field identification & locations
Identified by its deep opaque black color, waxy-to-vitreous luster, and high hardness capable of scratching glass. Found worldwide, with significant deposits in Brazil, India, Madagascar, and the United States.
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