Amethyst druse. A rock cavity lined with amethyst crystals. Amethyst is a type of quartz (silicon oxide) crystal which is purple in colour due to the presence of trace amounts of ferric iron. A druse is formed by deposition of silica on the inner surface of a geode (rock cavity). The silica precipitates out of water passing through the geode and forms quartz crystals. Quartz is one of the commonest minerals of the earth's crust (12 percent by volume). This sample is from Madagascar.