gemstone

Black Onyx

Silicon Dioxide (SiO2) - Chalcedony variety

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs scale), Color: Jet black, Luster: Vitreous to silky when polished, Crystal structure: Hexagonal (trigonal) cryptocrystalline, Cleavage: None, Fracture: Conchoidal, Specific Gravity: 2.65-2.667

Hardness: 6

Identified July 12, 2026More gemstone →

Explore Black Onyx in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed in the gas cavities of volcanic lava. It occurs through the slow deposition of silica from water at low temperatures. It is found in igneous, metamorphic, and sedimentary environments.

Uses & applications

Primarily used in jewelry (beads, cabochons, carvings) and ornamental items. Historically used for seals, intaglios, and protection amulets.

Geological facts

Natural solid black onyx is rare; most black onyx sold today is actually gray chalcedony that has been dyed black using a boiling sugar and sulfuric acid solution, a process used since ancient Roman times.

Field identification & locations

Identify by its hardness (will not be scratched by a steel knife) and its deep, opaque black color. Commonly sourced from Brazil, India, Madagascar, and the USA. Collectors look for uniform color and high polish.