mineral

Onyx (Black Onyx)

Onyx (Silicon Dioxide, SiO2)

Hardness: 6.5-7 (Mohs scale), Color: Black (often dyed), Luster: Vitreous to silky, Crystal Structure: Hexagonal (Trigonal) cryptocrystalline, Cleavage: None, Specific Gravity: 2.65-2.66

Hardness: 6

Identified May 1, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs scale), Color: Black (often dyed), Luster: Vitreous to silky, Crystal Structure: Hexagonal (Trigonal) cryptocrystalline, Cleavage: None, Specific Gravity: 2.65-2.66

Formation & geological history

Formed through the deposition of silica in gas cavities (vesicles) in lava. This process occurs in volcanic and sedimentary environments where silica-rich fluids cool over long periods.

Uses & applications

Primary use in jewelry (pendants, rings), decorative carvings, intaglios, and as a gemstone for protective amulets.

Geological facts

Black onyx in jewelry is almost always natural grey chalcedony that has been dyed black to enhance its depth and consistency. It was highly prized in ancient Rome for seals because wax wouldn't stick to it.

Field identification & locations

Identify by its waxy-to-glassy luster, extreme hardness (it will scratch glass), and cooling touch on the skin compared to plastic imitations. Commonly found in Brazil, India, and Madagascar.