Celestite

Celestite, also called celestine, is a sulfate mineral, strontium sulfate (SrSO₄), often found as pale blue crystals. Celestite (also called celestine) is a strontium sulfate mineral that forms in sedimentary environments - particularly in evaporite deposits, limestone cavities, and as a replacement mineral in fossils. The pale blue color comes from natural irradiation of trace impurities during the crystal's formation.

The world's most spectacular celestite specimens come from Sakoany, Madagascar, where sky-blue crystal clusters line geode cavities within sedimentary rock. These geodes, some exceeding a meter across, contain sharply terminated crystals with exceptional transparency. The Crystal Cave at Put-in-Bay, Ohio, contains the world's largest known celestite geode - a room-sized cavity lined with celestite crystals up to 45 centimeters long.

Industrially, celestite is the primary ore of strontium, which is used in fireworks (strontium compounds burn bright red), ceramic magnets, and specialty glass for cathode ray tube screens.

Celestite is identified by its pale blue color, orthorhombic crystal habit (tabular or prismatic crystals), and high specific gravity (3.97 - noticeably heavy for a pale mineral). It has perfect cleavage in three directions.

Distinguish from blue calcite (lower SG, acid reaction), blue barite (very similar but even denser at SG 4.50), and angelite (opaque, massive vs crystalline). Celestite's combination of sky-blue color, high density, and well-formed crystal habit is diagnostic. It does not fluoresce under UV light, which separates it from some similar minerals.