Agate is an improved variety of chalcedony and comprises the following kinds.

Banded or ribbon agate, running in delicate parallel layers.

Eye agate, forming concentric rings with a dark centre, giving the appearance of a human eye.

Fortification agate, running in circular parallel zigzag lines like the walls of a fortress.

Rainbow agate is a thin or concentric structure which when cut across and held towards the light shows an iridescence. Moss agate, light-gray to white and translucent to opaque agates, display black tracings like fine moss or trees. Mocha or tree agates are covered with black, brown, or red figures, as of trees and plants.

Beckite or silicified coral shells, silicified wood, wood agate, wood opal, cloudy agate, and agate jasper are some of the many varieties of this class.

The common carnelians, blood-stones, and onyxes are usually counted among the agates.

Uruguay, Brazil, Oberstein, Silesia, Surinam, India, Arabia, Saxony, Scotland, the United States, and Canada are the principal places where agates are found.