Celestite, also known as celestine, is a mineral composed of strontium sulfate. Its name is derived from the Latin word caelestis, meaning "heavenly," due to its often delicate, sky-blue color.

Celestite is a major source of strontium, used in producing fireworks and flares (due to its bright red flame), ceramics and glass, and compounds for electronics and medical applications.

Celestine crystals are found in some geodes. The world's largest known geode, a celestine geode 35 feet (11 m) in diameter at its widest point, is located near the village of Put-in-Bay, Ohio, on South Bass Island in Lake Erie. The geode has been converted into a viewing cave, Crystal Cave, with the crystals which once composed the floor of the geode removed. The geode has celestine crystals as wide as 18 inches (46 cm) across, estimated to weigh up to 300 pounds (140 kg) each.