Celestite

Celestite (also called celestine) is a strontium sulfate, SrSO4, crystallizing in the orthorhombic system as tabular or prismatic crystals within the barite group, alongside barite and anglesite. Its name, from the Latin caelestis (celestial), celebrates the soft sky-blue tone for which the species is best known, though colorless, white, orange and pale green material also occurs. With a Mohs hardness of only 3–3.5 and perfect cleavage, it is prized purely as a display and cabinet mineral rather than for jewelry.

The type locality is Bell's Mill, Bellwood, Blair County, Pennsylvania, but the species is best known today for the enormous geodes of Sakoany, near Katsepy in the Mahajanga Province of Madagascar, where cavities lined with gemmy sky-blue crystals can reach metric size. Sicily, Italy — with historic occurrences at Agrigento, Cattolica, Floristella and Caltanissetta — yields fine colorless to pale blue crystals on native sulfur and gypsum. Other classic sources include Clay Center and Crystal Cave in Ohio, Dundas in Ontario, Tarnobrzeg in Poland, and the Hammam-Zriba mine in Tunisia, noted for rare reddish crystals.

Many specimens show blue-white fluorescence under shortwave UV, and celestite remains the world's chief ore of strontium. For collectors, it offers a rare combination of delicate celestial color, well-formed orthorhombic crystals and striking evaporite associations with sulfur, gypsum and calcite.