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 recognized mechanism of celestine precipitation.

Celestite (SrSO4) also known as Celestine is a naturally occurring form of strontium sulfate. It is very similar to Barite (barium sulfate), but less common. Celestite occurs in sedimentary rock formations, especially in dolomite and dolomitic limestone. Celestite is mined industrially as a main source of strontium. Celestite also forms with other colorful minerals, making very nice combinations. Bright yellow sulfur with blue Celestite is one of the most famous colorful combinations of minerals. The ideal composition of Celestite is: SrO 56.42 % SO3 43.58 %.