Structure : Trigonal
Hardness : 7
Specific Gravity : 2.61
Refractive index : 1.53 - 1.5
Doubly Refracting (bi-refraction) : 0.004
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.
Onyx is very similar to agate. Onyx has straight bands of brown, white or black; agate has curved lines of many colors.
Since ancient times, Onyx has been dyed to improve it's color. Often treated agate is sold as Black Onyx. Onyx may chip or scratch rather easily. Onyx is found worldwide.