Epidote is an orthosilicate mineral, hydrated calcium iron aluminum silicate (Ca₂Fe₃(Al₂O)(OH)(Si₂O₇)[Si₂O₄]). It is found in metamorphic and igneous rocks as monoclinic system prismatic crystals and fibrous or granular masses; it is typically pistachio green, glassy, and brittle. Certain large crystals, 7.5–25 centimeters (3–10 inches), are collector's items.

Hardness 6–7; relative density 3.4.