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. Black Onyx sold
today is almost always clear agate that has been dyed black.
