Fluorite
Composition: CaF₂. Crystal system: isometric. Hardness: 4. Specific gravity: 3.0 to 3.25. Luster: vitreous. Color: violet, blue, colorless, green, yellow, brown, rose, and crimson red. Streak: white. Cleavage: four directions, octahedral, perfect. Fracture: subconchoidal to splintery. Tenacity: brittle. Diaphaneity: transparent to subtranslucent. Refractive index: 1.434.
Very fine green, transparent fluorite has been found near Voca, Mason County. The fluorite occurs as vug fillings in pegmatites, associated with crystals of pink microcline and colorless quartz. Most of the vugs have been completely filled by the fluorite; therefore, crystals ([fig. 15]) of the fluorite are not too common. Masses of fluorite several pounds in weight, rich green, and quite transparent have been found near Voca. Transparent pieces an inch or more in diameter are common.
Fig. 15. Common crystal form of fluorite.
Fluorite is much too soft for everyday use in jewelry and because of the low refractive index does not yield brilliant faceted stones. The perfect four-directional cleavage, relative softness, and brittle tenacity of the mineral make it difficult to facet. Faceted stones are seldom seen outside of collections. Cabochons are also difficult to cut from this material, but the rich color obtained is ample reward for the time and care necessary in cutting.
Fluorite occurs at several other localities in Texas, notably in Hudspeth, Brewster, Presidio, Llano, and Burnet counties, but not commonly in gem quality or colors that warrant its use as gem material.