Onyx may be porous, so care must be taken not to immerse the gem in substances that could change its appearance.
A damp, soft cloth, or scrubbing with a soft-bristle toothbrush, are the best ways to clean these two forms of quartz
Carved onyx, such as intaglios or cameos, may be collectible if they have provenance that can be identified as typical of a certain era. Signed gems from a known lapidary artist are also collectible.
Color: Onyx can be dyed in practically all colours, due to its porous nature
Much of the black chalcedony sold on the market is dyed black
In banded material, some bands may be dyed while others remain white
Black, also occuring in larger slabs with straight parallel layers of different colors
Translucent to opaque
Due to its lack of transparency Onyx is usually cut in cabochons or carved