mineral/sedimentary

Black Onyx

Cryptocrystalline Quartz (SiO2) with carbon/iron impurities

AI-generated identification · may be incorrect

Hardness: 6.5-7 Mohs; Color: Jet black often with white bands; Luster: Vitreous to waxy (when polished); Crystal Structure: Trigonal/Microcrystalline; Cleavage: None/Conchoidal fracture; Specific Gravity: 2.6-2.65.

Hardness: 6
Color: Jet black often with white bands
Luster: Vitreous to waxy (when polished)

Identified June 25, 2026More mineral/sedimentary →

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Physical properties

Formation & geological history

Onyx forms through the deposition of silica in gas cavities (vesicles) within volcanic rocks or in cold-solution environments. The black color usually results from the presence of carbon or iron minerals within the silicon dioxide layers.

Uses & applications

Widely used in jewelry for cabochons, beads, and cameos. Used in ornamental carvings, decorative arts, and as a popular tumbled stone for collectors and metaphysical practices.

Geological facts

Historically, black onyx was used by ancient Romans to make seals for letters, as the wax would not stick to the stone. Many natural black onyx stones on the market are actually gray chalcedony that has been dyed or heat-treated to achieve a deep black hue.

Field identification & locations

Identify in the field by its smooth, waxy texture and characteristic white parallel banding (unlike the concentric bands of agate). Found worldwide, with major deposits in Brazil, India, Madagascar, and the US (California/Arizona).