Rock Identifier
Black Onyx (Silicon dioxide (SiO2) with parallel banded structure) — Mineral (variety of chalcedony)
Mineral (variety of chalcedony)

Black Onyx

Silicon dioxide (SiO2) with parallel banded structure

Hardness: 6.5–7 on Mohs scale; Color: Solid black (often treated) or black with white bands; Luster: Vitreous to silky; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Specific Gravity: 2.60–2.65

Hardness
6
Color
Solid black (often treated) or black with white bands
Luster
Vitreous to silky
Identified More mineral (variety of chalcedony) →

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Physical properties

Hardness: 6.5–7 on Mohs scale; Color: Solid black (often treated) or black with white bands; Luster: Vitreous to silky; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Specific Gravity: 2.60–2.65

Formation & geological history

Formed in the gas cavities (vesicles) of volcanic lava. As silica-rich water percolates through the rock, it deposits layers of chalcedony. It can be found in rocks from various geological ages, often in prehistoric volcanic deposits.

Uses & applications

Predominantly used in jewelry, beads, cameos, and small ornamental carvings. Historically used for seals and intaglios.

Geological facts

Natural solid black onyx is extremely rare; most commercial 'black onyx' is actually grey chalcedony that has been dyed or sugar-treated and carbonized with acid to achieve a deep black color. In ancient times, it was believed to provide protection and strength to the wearer.

Field identification & locations

Identify in the field by its waxy luster, lack of cleavage, and extreme hardness (cannot be scratched by a steel knife). Common locations include Brazil, India, Madagascar, and the United States. For collectors, look for smoothness and depth of color.

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