Agate is a rock consisting primarily of cryptocrystalline silica, chiefly chalcedony, alternating with microgranular quartz. It is characterized by its fineness of grain and variety of color. Explore Art By God’s collection of Agate at affordable prices in a variety of colors and sizes. Gorgeous Agate for sale. Agate Bookends, Agate Slices, Agate Gemstones, Agate Minerals and Agate Wind chimes.