gemstone

Black Onyx

Chalcedony (SiO2)

Hardness: 6.5-7 on the Mohs scale, Color: Black (often artificially treated, though natural black chalcedony exists), Luster: Vitreous to waxy, Crystal Structure: Trigonal (microcrystalline quartz), Cleavage: None, Specific Gravity: 2.60-2.65

Hardness: 6

Identified August 2, 2026More gemstone →

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Physical properties

Formation & geological history

Formed in the gas cavities of volcanic and sedimentary rocks where silica-rich groundwater deposits silica in regular layers.

Uses & applications

Used extensively in jewelry, ornamental carvings, cameos, and beads.

Geological facts

Black onyx has been used for signet rings and intaglios since ancient Roman and Greek times due to its ability to hold a fine polish and contrast sharply when carved.

Field identification & locations

Identified by its deep black color, microcrystalline texture, conchoidal fracture, and glassy to waxy luster. Common sources include Brazil, India, Madagascar, and the USA.