Onyx

About it

Chalcedony with straight, parallel bands, classically black and white, used for cameos. Most black onyx today is dyed chalcedony; Mexican onyx is banded calcite, a different and much softer stone.

Type

Variety

Group

Quartz variety (chalcedony)

Formula

SiO2

Hardness (Mohs)

6.5–7

Specific gravity

2.57–2.64

Streak

white

Lustre

waxy, vitreous (glassy)

Crystal system

Trigonal

Colours

black and white, black, white, brown

Transparency

translucent, opaque

Cleavage and fracture

none; conchoidal to uneven fracture

Typical values: real specimens vary. For more, see mindat.org.