gemstone
Black Onyx
Chalcedony (SiO2)
Hardness of 6.5 to 7 on the Mohs scale, color is solid black, vitreous to waxy luster, microcrystalline (cryptocrystalline) quartz structure, conchoidal fracture with no cleavage, specific gravity of 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, vitreous to waxy luster, microcrystalline (cryptocrystalline) quartz structure, conchoidal fracture with no cleavage, specific gravity of 2.60 to 2.65.
Formation & geological history
Formed by the deposition of silica in gas cavities of volcanic lavas or groundwater solutions infiltrating cavities in rocks, predominantly through low-temperature hydrothermal processes.
Uses & applications
Widely used in jewelry such as rings, necklaces, and brooches, as well as for carving cameos, beads, and ornamental objects.
Geological facts
Black onyx has been used since ancient Egyptian and Roman times for signet rings and protective amulets. Natural solid black chalcedony is relatively rare, so most commercial black onyx is color-enhanced gray or brown chalcedony.
Field identification & locations
Identified by its deep opaque black color, waxy luster, and high scratch resistance due to its quartz composition. Commonly found in Brazil, India, Madagascar, and the USA.
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