Epidote is a calcium aluminium iron sorosilicate mineral. The name is derived from the Greek word "epidosis" (επίδοσις) which means "addition" in allusion to one side of the ideal prism being longer than the other.Well-developed crystals of Epidote, Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing in the monoclinic system, are of frequent occurrence: they are usually prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often deeply striated and crystals are often twinned. Many of the characters of the mineral vary with the amount of iron present, for instance the color, the optical constants, and the specific gravity. The color is green, grey, brown or nearly black, but usually a characteristic shade of yellowish-green or pistachio-green. It displays strong pleochroism, the pleochroic colors being usually green, yellow and brown. Epidote is also almost identical in composition to Clinozoisite but lacks significant iron in its structure.
Some of the most outstanding Epidote crystals come from Pakistan, in the Tormiq valley, Skardu District, and in Alchuria and Hachupa in the Shigar Valley. Noteworthy localities also include the Dark, chisel-shaped crystals associated with Prehnite from the Kayes Region in Mali, or the famous specimens from Green Monster Mountain, on Prince of Wales Island in Alaska.