Agate

Agate, a mineral composed of layers of quartz, generally of different varieties of colours, intimately joined together. It usually occurs as rounded nodules or veins in trap rock. The layers are often concentric, and in the section sometimes appear nearly circular. The composition varies greatly, but silica is always predominant, usually with alumina and oxide of iron. Chalcedony (q.v.) carnelian, amethyst, common quartz, jasper, opal, and flint occur as layers in agate, and are spoken of as kinds of agate. There are also clouded agates, star-agates, moss-agates, &c., so called from their appearance. It takes a fine polish, and is much used for ornamental purposes. Many agates are found in Scotland, and are sold under the name of Scotch Pebbles.