Celestine or celestite (SrSO4) is a mineral consisting of strontium sulfate. The mineral is named for its occasional delicate blue color. Celestine is the principal source of the element strontium, commonly used in fireworks and in various metal alloys.
Celestine 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.
The skeletons of the protozoan Acantharea are made of celestine, unlike those of other radiolarians which are made of silica.
In carbonate marine sediments, burial dissolution is a recognised mechanism of celestine precipitation.