celestite or celestine, mineral appearing in blue-tinged or white orthorhombic crystals or in fibrous masses. The natural sulfate of strontium, SrSO₄, it is important as a source of strontium and of certain of its compounds, e.g., strontium hydroxide, used in refining beet sugar, and strontium nitrate, used in red signal flares. It occurs in England, in Sicily, and in the United States on islands in Lake Erie and also in Pennsylvania, New York, and Ohio.