Onyx

Onyx is a rich-looking, affordable black stone. It is often mixed with pearls to create elegant jewelry. Onyx, sard, and sardonyx are all varieties of chalcedony (microcrystaline 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.