mineral

Black Onyx

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs scale), Color: Jet black, Luster: Vitreous to silky (when polished), Crystal structure: Trigonal/Monoclinic Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.65-2.677

Hardness: 6

Identified June 30, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs scale), Color: Jet black, Luster: Vitreous to silky (when polished), Crystal structure: Trigonal/Monoclinic Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.65-2.677

Formation & geological history

Formed by the deposition of silica in gas cavities (vesicles) within volcanic lavas. It is a chalcedony variety that develops parallel bands, though many polished commercial stones are uniformly black due to treatment.

Uses & applications

Extensively used in jewelry (rings, beads, pendants), ornamental carvings, cameos, and as a popular grounding stone in metaphysical collections.

Geological facts

Natural solid black onyx is actually rare; most commercial black onyx is natural gray chalcedony that has been dyed black using a solution of sugar and sulfuric acid, a treatment method stabilized since ancient Roman times.

Field identification & locations

Identified by its waxy to vitreous luster and lack of grain. In the field, look for volcanic regions with agate deposits. Collectors should look for even color and high polish without visible pits or cracks.