A mineral with the chemical composition SrSO₄. Celestite occurs commonly in colorless to sky-blue, orthorhombic, tabular crystals. Fracture is uneven and luster is vitreous. Hardness is 3–3.5 on Mohs scale and specific gravity is 3.97. It fuses readily to a white pearl. It is only slowly soluble in hot concentrated acids or alkali carbonate solutions. The strontium present in celestite imparts a characteristic crimson color to the flame.