Agate, AG iht, is a banded form of chalcedony, a fine-grained, porous type of quartz. It occurs primarily as layers in the cavities of sedimentary rocks (see Sedimentary rock). Most types of agates are dully colored. Their bands vary from white through gray to black. In some cases, the bands may be pale red, yellow, or blue. The colors result from the presence of such impurities as iron oxide and manganese oxide. Agates differ in the pattern of their bands. Onyx is a type of agate distinguished by parallel bands that lie in a plane. The bands of eye agate form circles that spread out from the center. Moss agate has delicate mosslike patterns.