Chalcedony (microcrystalline quartz)
Black Onyx
Onyx is banded chalcedony - the same layered silica structure as agate, but with the bands lying in flat parallel planes rather than concentric rings. Uniformly black material is uncommon in nature, so most black onyx on the market is pale chalcedony darkened by a long-established sugar-and-acid treatment that deposits carbon in its pores.
- Mineral family
Chalcedony (microcrystalline quartz)
- Typical colours
Even black, with charcoal and slate greys where the light lands
- Hardness (Mohs)
6.5 - 7
- Luster
Vitreous when polished, soft and chalky when matte
- Texture
Very fine-grained and completely opaque, taking a deep glassy polish that shows every fingerprint.
Caring for it
Wipe with a soft dry or barely damp cloth. Keep it away from solvents, bleach and abrasive polishes, since the darkened surface layer is thin, and store it apart from harder stones to protect the shine.