mineral

Black Onyx

Silicon Dioxide (SiO2) with carbon impurities

Hardness: 6.5-7 (Mohs); Color: solid black; Luster: Vitreous to waxy; Crystal structure: Trigonal/Monoclinic; Cleavage: None; Specific gravity: 2.65-2.667

Hardness: 6
Color: solid black
Luster: Vitreous to waxy

Identified May 23, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs); Color: solid black; Luster: Vitreous to waxy; Crystal structure: Trigonal/Monoclinic; Cleavage: None; Specific gravity: 2.65-2.667

Formation & geological history

Formed in the gas cavities of volcanic lava flows (vesicles) through the deposition of silica at low temperatures from groundwater. It often occurs in ancient volcanic regions spanning hundreds of millions of years.

Uses & applications

Used primarily in jewelry (rings, beads, pendants), ornamental carvings, intaglios, and increasingly for metaphysical uses or small decorative objects.

Geological facts

The name comes from the Greek word 'onyx', meaning nail or claw, because of a myth that Cupid clipped Venus' fingernails and the gods turned them into stones. Naturally solid black onyx is actually quite rare; most commercial black onyx is heat-treated or dyed gray chalcedony.

Field identification & locations

Look for a smooth, waxy surface and uniform deep black color. In the field, look for chalcedony nodules in volcanic rock suites. It can be distinguished from obsidian by its higher hardness and lack of sharp conchoidal edges.

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