mineral

Black Onyx

Silicon Dioxide (SiO2)

Hardness: 6.5-7 Mohs, Color: Deep black (often dyed for uniform color), Luster: Vitreous/Waxy, Structure: Cryptocrystalline, Cleavage: None

Hardness: 6
Luster: Vitreous/Waxy, Structure: Cryptocrystalline, Cleavage: None

Identified July 14, 2026More mineral →

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Physical properties

Hardness: 6.5-7 Mohs, Color: Deep black (often dyed for uniform color), Luster: Vitreous/Waxy, Structure: Cryptocrystalline, Cleavage: None

Formation & geological history

Formed through the deposition of silica in gas cavities (vesicles) in volcanic lava, leading to the distinctive bands often found in chalcedony. Many solid black specimens are historically enhanced by soaking in sugar solutions and then heating in acid.

Uses & applications

Widely used in jewelry (rings, beads, intaglios), ornamental carvings, and historical talismans.

Geological facts

While naturally occurring black onyx exists, most solid black onyx sold today is actually gray chalcedony that has been dyed to achieve a uniform, deep black appearance.

Field identification & locations

Identify by its waxy luster, lack of transparency (opaque), and high hardness. In the field, look for volcanic rock nodules. Common sources include Brazil, India, and Madagascar.