Chalcedony is a cryptocrystalline form of silica, a big word basically meaning it has very small fine crystals. The result is a stone which has a waxy lustre, and may be semitransparent or translucent. Its colour is usually white to gray, grayish-blue or a shade of brown ranging from pale to nearly black.
Other shades have been given specific names: A clear red chalcedony is known as carnelian or sard. A green variety colored by nickel oxide is called chrysoprase. Prase is a dull green. And last but not least, Onyx has flat black and white bands.