Epidote

Epidote is a calcium aluminum iron silicate that forms in regional and contact metamorphic rocks, particularly in greenschist facies conditions. Its name comes from the Greek 'epidosis' meaning 'increase' - referring to the crystal's base having one side longer than the other.

Epidote's distinctive pistachio to dark olive green color comes from iron (Fe³⁺) in its structure. The more iron present, the darker the green - the finest gem-quality specimens are deeply colored but still translucent. Austrian specimens from the Knappenwand locality in the Hohe Tauern mountains are considered among the finest epidote crystals ever found.

Epidote is geologically important as a key indicator mineral in metamorphic petrology. Its presence tells geologists about the pressure, temperature, and chemistry of the metamorphic event that formed the rock. It's also the green component in unakite (epidote + pink feldspar + quartz).

Epidote is identified by its distinctive pistachio-to-olive green color, monoclinic prismatic crystals with a characteristic elongated habit, and hardness of 6-7. The green is distinctly different from the greens of tourmaline, emerald, or jade.

Distinguish from green tourmaline (different crystal cross-section), actinolite (fibrous habit, different crystal system), and olivine (orthorhombic, different green tone). Epidote's one-sided elongation and specific pistachio green are diagnostic.