Microcrystalline variety of the quartz group of varying colors, a chalcedony. Agate forms when silicated rich water seeps into crevices and becomes trapped by cooling magma. It also occurs as spherical or almond shaped nodules in volcanic rocks. The banding is the result of rhythmic crystallization. Frequently in the center of the almond shaped rock are well developed crystals. Colors may vary but the most common is varying shades of grey. There are numerous varieties of agate. Agates range from transparent to opaque in a variety of beautiful colors. Agate presents various tints in the same specimen. The stones can be artificially stained to produce combinations of color more vivid than those found in the natural state.

Varieties: There are several types of agate. Varieties include Blue Agate, Blue Lace Agate, Crazy Lace Agate, Green Agate, Indian Agate, Moss Agate, Tree Agate and Wood Agate. The Moss agate or mocha stone contains visible impurities in the form that resemble moss.

Astrology: Agate is the birthstone of Taurus (Bull): April 20 - May 20.