There is only one well-authenticated find of diamond in Texas. A small brownish diamond was found in 1911 on section 64, block 44, Foard County (Sterrett, 1912, pp. 1040-1041). The exact weight of the stone has not been recorded, but one authority estimated that it was of sufficient size and clarity to yield a cut stone of about one-quarter carat.
The only diamond-bearing rocks known in the United States are in Pike County, Arkansas. Although many other diamonds have been found in the United States, all were loose in gravels or streams except for some stones at the Arkansas locality. The fact that one diamond was found in Foard County does not mean that the prospects of finding more diamonds in Texas are much better there than anywhere else in the State. It is highly unlikely that more than a very few diamonds will ever be found in Texas, and any stones that may be found in the future are likely to be widely scattered.
Epidote
Composition: HCa₂(Al, Fe)₂Si₃O₁₃. Crystal system: monoclinic. Hardness: 6 to 7. Specific gravity: 3.25 to 3.5. Luster: vitreous. Color: yellowish green to brownish green and brown. Streak: uncolored to grayish. Cleavage: two directions. Fracture: uneven. Tenacity: brittle. Diaphaneity: transparent to opaque. Refractive index: about 1.72 to 1.77.
Llano County has furnished some green and brownish-green epidote that is suitable for cutting into cabochons. Most of the material that approaches gem quality has come from contact metamorphic zones and is associated with garnet, quartz, and some scheelite. Some small cavities in the rocks contain tiny transparent crystals of gem quality, but the largest obtainable flawless faceted stones would probably be less than 15 points.
Faceted stones of epidote are sometimes known as pistacite owing to their common pistachio-green color.
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.
