Celestine

Named for its occasional sky-blue color, celestine is a strontium sulfate (SrSO4) and the primary ore of strontium. It crystallizes in the orthorhombic system as tabular or prismatic crystals with a vitreous luster, a white streak, and perfect {001} cleavage. The mineral is transparent, ranges from colorless through pale blue, white, and yellow, and has a Mohs hardness of 3 to 3.5 with a specific gravity near 3.96. Its most famous occurrence is the Crystal Cave at Put-in-Bay, Ohio, the world's largest geode, whose interior is lined with celestine crystals reaching 90 centimeters in length. Its close resemblance to barite reflects a shared sulfate structure, though celestine is distinctly less dense.

Celestine (celestite) is the primary ore of strontium, named for its occasional celestial blue color. The world's largest geode, the Crystal Cave in Put-in-Bay, Ohio, is lined with celestine crystals up to 90 cm long.