Celestite
Composition: SrSO₄. Crystal system: orthorhombic. Hardness: 3.0 to 3.5. Specific gravity: 3.95 to 3.98. Luster: vitreous. Color: white, blue, greenish, reddish, and brownish. Streak: white. Cleavage: three directions, although one of these directions is not easily developed. Fracture: uneven. Tenacity: brittle. Diaphaneity: transparent to subtranslucent. Refractive index: 1.62 to 1.63. Dispersion: moderate.
Celestite is very seldom cut into gems. Being very soft, brittle, and having three cleavages, celestite is completely unsuitable for jewelry. These same properties make this mineral exceedingly difficult to facet; however, faceted stones are seen in large collections.
Fig. 14. Common crystal form of Travis County celestite. Same crystal form as shown in [Plate I], A.
Fine crystals of colorless and blue gem-quality celestite ([Pl. I], A, and [fig. 14]) have been found at Mount Bonnell and other localities west of Austin, Travis County. The celestite crystals occur in vugs or geodes in limestone. The crystals are mostly white or colorless and fractured near the base or where attached, but the tips of the crystals are commonly clear celestine blue and completely free of flaws.
Crystals several inches in length have been found, but the average size is about 1 inch. The smaller crystals are frequently more transparent and consequently better suited for cutting. It is very difficult to obtain crystals that will allow the cutting of flawless stones of more than 4 or 5 carats.
Bluish and colorless celestite of gem quality and fine crystals have been found near Lampasas, Lampasas County, and near Brownwood, Brown County, but neither of these localities has been very productive of good gem material.
Celestite geodes have been found in parts of Coke, Fisher, and Nolan counties, but these geodes contain little gem material.