A semiprecious variety of chalcedonic silica
(Si02) in which the crystals are too fine to
be detected under a microscope, a state known
as cryptocrystalline. It has straight
parallel bands of different colours: milk-
white, black, and red. Sardonyx, an onyx
variety, has layers of brown or red carnelian
alternating with lighter layers of onyx. It
can be carved into cameos.