One of the most popular gemstones, agate is a cryptocrystalline variety of quartz. It is primarily composed of silica in the form of quartz, along with variable amounts of microcrystalline or cryptocrystalline quartz. It also contains variable amounts of limonite, chalcedony, and other minerals such as siderite, mica, orthoclase, and fluorite. Agate's banding patterns are caused by the way the silica layers are deposited during the stone's formation. The Unopened Agate Crystal appear as layers and swirls and are often an attractive feature of the stone.