mineral

Black Onyx

Silicon dioxide (SiO2) - Chalcedony variety

Hardness: 6.5-7 (Mohs); Color: Pure black (often dyed) or black with white banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None; Specific Gravity: 2.65-2.67

Hardness: 6
Color: Pure black (often dyed) or black with white banding
Luster: Vitreous to waxy

Identified March 20, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs); Color: Pure black (often dyed) or black with white banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None; Specific Gravity: 2.65-2.67

Formation & geological history

Formed through the deposition of silica in gas cavities (vesicles) in volcanic lava. Over time, layers of microcrystalline quartz create the banded structure characteristic of chalcedony. Foundations can date back millions of years depending on the host volcanic rock.

Uses & applications

Extensively used in jewelry making (beads, cabochons, carvings), decorative objects, and intaglio engravings. It is also a popular metaphysical stone.

Geological facts

Natural solid black onyx is extremely rare; most commercial black onyx is actually gray chalcedony that has been treated with a sugar-acid solution to achieve a deep, uniform black color. In ancient Rome, it was often used for seals because it did not stick to wax.

Field identification & locations

Identify in the field by its smooth, waxy texture and conchoidal (shell-like) fracture pattern. Commonly found in Brazil, India, Madagascar, and the USA. For collectors, look for high polish and absence of visible scratches.

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