mineral

Black Onyx

Chalcedony (Silicon Dioxide, SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Solid black (often treated) or banded black and white; Luster: Vitreous to waxy; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None; Specific Gravity: 2.65-2.667.

Hardness: 6
Color: Solid black (often treated) or banded black and white
Luster: Vitreous to waxy

Identified January 23, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the deposition of silica in gas cavities (vesicles) in volcanic lava, typically at low temperatures. Most solid black onyx sold today is actually gray chalcedony that has been dyed to achieve a uniform black color, a practice dating back to Roman times.

Uses & applications

Primary use is in jewelry (rings, beads, intaglios), ornamental carvings, and cameos. Historically used for making bowls and small vessels. In collecting, it is valued for its deep, opaque color and high polish.

Geological facts

Ancient Romans believed onyx gave courage and protected the wearer from harm. The name comes from the Greek word 'onux', meaning fingernail or claw, referring to the white bands in the stone that resemble a human nail.

Field identification & locations

Identify in the field by its waxy luster, conchoidal fracture, and lack of visible crystals. It is commonly found in Brazil, India, Madagascar, and the US (California). To distinguish from glass, check for coldness to the touch and its ability to scratch glass or mild steel.