_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.
[Illustration: 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.