_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.