Agate is a translucent variety of microcrystalline quartz. It is used as a semiprecious stone when it is of desirable quality and color.
Agate generally forms by the deposition of silica from groundwater in the cavities of igneous rocks. The agate deposits in concentric layers around the walls of the cavity, or in horizontal layers building up from the bottom of the cavity. These structures produce banded patterns that are characteristic of many agates. Some of these cavities are lined with crystals and those are known as geodes.

Agate occurs in a wide range of colors, which include brown, white, red, gray, pink, black, and yellow. The colors are caused by impurities and occur as alternating bands within the agate. The different colors were produced as groundwaters of different compositions seeped into the cavity. The banding within a cavity is a record of water chemistry change. This banding gives many agates the interesting colors and patterns that make it a popular gemstone.