Onyx, sard, and sardonyx are all varieties of chalcedony
(microcrystalline quartz). Onyx comes in brown, white, grey, and black.
Sard is a reddish-brown variety. Sardonyx is a blend of sard and onyx
with the red bands of sard and the white bands of onyx. Onyx, Sard and
Sardonyx all have a hardness rating of 7.
Onyx is very similar to agate. Onyx has straight bands of brown,
white or black; agate has curved lines of many colours.
Since ancient times, Onyx has been dyed to improve it's colour.