mineral

Black Onyx

Silicon Dioxide (SiO2)

Hardness: 6.5–7 on Mohs scale; Color: Black (often dyed or heat-treated for uniform color); Luster: Vitreous to silky when polished; Crystal structure: Trigonal/Monoclinic (Cryptocrystalline); Cleavage: None; Specific Gravity: 2.65–2.677.

Hardness: 6
Color: Black (often dyed or heat-treated for uniform color)
Luster: Vitreous to silky when polished

Identified March 9, 2026More mineral →

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Physical properties

Formation & geological history

Formed in the gas cavities (vesicles) of volcanic lava; as silica-rich fluids deposit in thin layers over long periods. It is found in many geological eras from the Precambrian to more recent volcanic deposits.

Uses & applications

Predominantly used in jewelry (rings, pendants, beads), cameos, intaglios, and decorative carvings. Historically used for making bowls and beads in ancient Egypt and Rome.

Geological facts

Natural solid black onyx is extremely rare; most commercial 'black onyx' is actually gray chalcedony that has been dyed black using a sugar-acid treatment or heat-treated. It was highly valued in Victorian 'mourning jewelry'.

Field identification & locations

Identify in the field by its waxy luster, conchoidal fracture, and hardness (it will scratch glass). Commonly found in Brazil, India, Madagascar, and the US. Collectors should look for uniform color and absence of visible inclusions under magnification.