Celestine is a mineral consisting of strontium sulfate (SrSO4). It is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources of the element strontium, commonly used in fireworks and in various metal alloys. The mineral occurs as crystals, and also in compact massive, and fibrous forms. It is found worldwide, mostly found in sedimentary rocks, usually in small quantities. Pale blue crystal specimens, as shown in this photograph, are found in Madagascar.Photograph credit: Ivar Leidus