What is Onyx?
Onyx is a banded chalcedony and quartz material, prized for clean parallel layers, strong contrast and polished surfaces that turn microscopic silica growth into a natural graphic pattern.
Mineralogy and crystal structure
Reference formula: SiO2. Onyx is a lapidary material based on banded chalcedony, a microcrystalline form of quartz. True onyx is a banded form of chalcedony or microcrystalline quartz, commonly described with the approximate formula SiO2. The material is not a separate mineral species and the banding does not create a new lattice. The displayed COD 1011097 image is a labelled representative quartz host model; commercial 'onyx marble' may instead be banded calcite and must not be conflated with silica onyx. The representative quartz host uses trigonal symmetry. Typical hardness is 6.5–7 on the Mohs scale and specific gravity is approximately 2.58–2.65.
Colour, lustre and habit
Typical colours include black, white, brown, red, grey. The usual banded, massive, microcrystalline habit and waxy, vitreous, dull lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Onyx forms in or is associated with silica-rich groundwater filling cavities and fractures, with rhythmic deposition of chalcedony and quartz layers. Important documented localities include Brazil, Uruguay, India, Mexico, Madagascar, Pakistan and Turkey.
Natural parallel lines for the lapidary market
Onyx sells through pattern discipline: parallel bands, crisp contrast and a polish that lets the layers read immediately. Strong provenance and a clear distinction between true silica onyx and calcite 'onyx marble' make the description more valuable than a generic colour claim.
Collecting and care
Protect polished surfaces from scratches, acids and sudden impact. Avoid vinegar and harsh cleaners, and state whether a piece is true silica onyx, sardonyx, dyed material or banded calcite sold as onyx marble.
