Celestite (SrSO4) or strontium sulfate, crystallizes as prismatic crystals, but sometimes also in tabular orthorhombic form, compact masses or fibrous forms. Celestite has a hardness of 3 to 3.5 on the Mohs scale and is quite fragile, even slightly soluble in water. Celstite is also known by the name of celestine. It has a perfect cleavage parallel to the base, and fractures easily.

Celestite is considered a beautiful mineral due to its transparent to translucent qualities, with always pale colors, such as colorless, white, pale blue and pale yellow. It also occurs in shades of orange, red and red-brown. These color qualities have made celestite a sought after gemstone by collectors worldwide. Celestite is so named because the pale blue variety is reminiscent of the color of the sky (heavens).

Celestite occurs in cavities of volcanic rocks as a byproduct of solutions rich in sulfides, chlorides, and carbonates. It also occurs more frequently in limestone rock caverns, and in veins, associated with other sulfide derived rocks such as galena and sphalerite.

Celestite is found in deposits worldwide, but never in abundance. The best pale blue specimens of celestite come from Madagascar, but there are also places such as Montecchio Maggiore in Vicenza, Italy that yield really fine blue celestite crystals.

The largest celestite crystals so far known are found in Put-in-Bay, Ohio, USA. These white celestite crystals measure 50-75 cm long, and may weigh up to 3 kilos. Put-in-Bay has the largest celestite cavern so far discovered by humans; it is a geode with big celestite crystals measuring as much as 45 cm and 130 kilos. The geode itself is impressive, measuring 10 meters across at its widest point.

Elsewhere, celestite deposits have been found to occur in Bristol (England), Tunisia, Russia, Ukraine, and Sicily (Italy). Historically, celestite has been used to make fireworks, as it provides the fireworks with a crimson flame, and it is used in the manufacture of glass, rubber, paint and batteries.