gemstone

Black Onyx

Chalcedony (Silicon Dioxide, SiO2)

Hardness: 6.5-7.0 (Mohs); Color: solid black (often dyed); Luster: vitreous to waxy; Crystal structure: Trigonal/Monoclinic (cryptocrystalline); Specific gravity: 2.65-2.677

Hardness: 6
Color: solid black (often dyed)
Luster: vitreous to waxy

Identified July 6, 2026More gemstone →

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Physical properties

Hardness: 6.5-7.0 (Mohs); Color: solid black (often dyed); Luster: vitreous to waxy; Crystal structure: Trigonal/Monoclinic (cryptocrystalline); Specific gravity: 2.65-2.677

Formation & geological history

Forms through the deposition of silica in gas cavities (vesicles) in volcanic lavas. Most black onyx sold commercially is natural gray chalcedony that has been treated with a sugar solution and then carbonized with sulfuric acid to achieve a consistent deep black.

Uses & applications

Extensively used in jewelry (rings, beads, pendants), ornamental carvings, intaglios, and as a balancing stone in metaphysical practices.

Geological facts

The name comes from the Greek word 'onyx', meaning fingernail or claw. In ancient times, it was popular for making cameos due to its banded nature, though solid black specimens have been prized since the Victorian era for mourning jewelry.

Field identification & locations

Identify by its hardness (will scratch glass), smooth waxy texture when polished, and lack of visible grain. Natural black onyx is rare; most field finds will be banded chalcedony or agate. Common sources include Brazil, Uruguay, India, and Madagascar.