mineral

Black Onyx

Chalcedony (SiO2 with inclusions)

Hardness: 6.5-7 (Mohs), Color: Black/dark grey with potential banding, Luster: Vitreous/waxy, Crystal Structure: Trigonal/Monoclinic (Cryptocrystalline), Cleavage: None, Specific Gravity: 2.58-2.65

Hardness: 6

Identified May 31, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs), Color: Black/dark grey with potential banding, Luster: Vitreous/waxy, Crystal Structure: Trigonal/Monoclinic (Cryptocrystalline), Cleavage: None, Specific Gravity: 2.58-2.65

Formation & geological history

Formed in the gas cavities or vesicles of volcanic rocks as silica precipitates from water over long periods. Much commercial black onyx is treated gray chalcedony.

Uses & applications

Extensively used in jewelry (rings, beads, cameos), decorative carvings, and as a healing stone in metaphysical communities.

Geological facts

While true onyx has parallel bands, the name is commonly used for solid black dyed chalcedony. It has been used for seals and intaglios since ancient Roman times.

Field identification & locations

Identify by its smooth, waxy surface and hardness (it will scratch glass). Often found in Brazil, India, and Madagascar. Collectors look for high-polish and consistency of color.