mineral

Black Onyx

Black Chalcedony (SiO2)

Hardness: 6.5–7 on Mohs scale; Color: Jet black (often dyed for uniform color); Luster: Vitreous to waxy; Crystal structure: Hexagonal (trigonal); Cleavage: None; Specific gravity: 2.60–2.65

Hardness: 6
Color: Jet black (often dyed for uniform color)
Luster: Vitreous to waxy

Identified March 15, 2026More mineral →

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Physical properties

Hardness: 6.5–7 on Mohs scale; Color: Jet black (often dyed for uniform color); Luster: Vitreous to waxy; Crystal structure: Hexagonal (trigonal); Cleavage: None; Specific gravity: 2.60–2.65

Formation & geological history

Formed in the gas cavities (vesicles) of volcanic rocks like basalt. Silica-rich waters deposit layers of microcrystalline quartz over thousands to millions of years. Most black onyx used in jewelry has been treated with sugar-acid solutions to achieve deep black uniformity.

Uses & applications

Predominantly used in jewelry (rings, brooches, cameos), decorative carvings, and small ornamental objects. In historical contexts, it was widely used for Intaglio and cameo relief carving.

Geological facts

Ancient Romans believed onyx gave power and protected the wearer during battle. The word 'onyx' comes from the Greek word for 'fingernail' or 'claw,' referring to the banded variety's resemblance to a human nail. While natural solid black onyx exists, it is extremely rare; most modern black onyx is actually gray chalcedony that has been dyed.

Field identification & locations

Identify in the field by its smooth, conchoidal fracture and lack of banding (if solid). It is commonly found in Brazil, India, Madagascar, and the US. For collectors, check for heat sensitivity and scratch resistance against steel to differentiate from glass or plastic imitations.