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.