mineral

Black Onyx

Cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 Mohs; Color: Jet black; Luster: Vitreous to silky; Crystal structure: Hexagonal (trigonal); Cleavage: None; Specific gravity: 2.65-2.677

Hardness: 6
Color: Jet black
Luster: Vitreous to silky

Identified April 9, 2026More mineral →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 6.5-7 Mohs; Color: Jet black; Luster: Vitreous to silky; Crystal structure: Hexagonal (trigonal); Cleavage: None; Specific gravity: 2.65-2.677

Formation & geological history

Forms in the gas cavities or geodes of volcanic lava. It occurs through the slow deposition of silica from groundwater into cavities. Most black onyx on the market today is actually heat-treated and dyed chalcedony.

Uses & applications

Primarily used in jewelry (rings, beads, pendants), ornamental carvings, and cameos. Historically used for protective amulets.

Geological facts

While natural black onyx exists, it is exceptionally rare; most commercial 'black onyx' is natural grey chalcedony that has been dyed black to achieve color uniformity. It has been used for centuries in mourning jewelry and signet rings.

Field identification & locations

Identify by its smooth, waxy luster and consistent black color. Natural specimens may show banding. It is cool to the touch. Commonly sourced from Brazil, India, Madagascar, and the USA.