Olivine, iron magnesium silicate, is common throughout the world and is one of the first minerals to form in a cooling lava flow. It belongs to the chrysolite group of minerals, which include forsterite (magnesium silicate) and fayalite (iron silicate). Olivine is a mixture of these two species. Excellent crystals occur in a number of localities, including a remote island in the Red Sea near Egypt that has been known variously as Zebirget, Zabargad and St. John's Island. The olivine crystals on this island are transparent and a rich green color, allowing them to be cut as gemstones. The gem form of olivine is called peridot, and is the August birthstone.