Celestite (SrSO4) is a mineral consisting of strontium sulfate. The mineral is also known as celestine and is named for its occasional delicate blue color. Celestite occurs as crystals, and also in compact massive and fibrous forms. It is mostly found in sedimentary rocks, often associated with the minerals gypsum, anhydrite, and halite. The mineral is found worldwide, usually in small quantities. Pale blue crystal specimens are found in Madagascar.