Agates have been used as gemstones for thousands of years. They were some of the earliest stones fashioned by people. Today, they are cut into cabochons, beads, small sculptures, and functional objects such as paperweights and bookends.
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 the banded patterns that are characteristic of many agates.
Agate occurs in a wide range of colors, which include brown, white, red, gray, pink, black, and yellow.
Information courtesy of Hobart M. King, Ph.D., GIA Graduate Gemologist