Piemontite

Piemontite is the manganese-bearing variety of epidote, where manganese (Mn³⁺) substitutes for aluminum and iron in the crystal structure to produce a distinctive deep red to reddish-brown color. It forms during regional metamorphism of manganese-rich sedimentary rocks, particularly in the greenschist to amphibolite facies.

The mineral was first described from the Piedmont (Piemonte) region of northwestern Italy, giving it its name. It occurs as prismatic crystals or massive aggregates in schists and quartzites that originally contained manganese-rich sediments.

Piemontite is not particularly rare as a mineral, but gem-quality transparent material is extremely scarce. The deep red color is among the richest of any naturally occurring silicate mineral. In thin section under a microscope, piemontite shows pleochroism from red to yellow to purple, making it one of the most visually dramatic minerals in petrography.

Piemontite is identified by its deep red to reddish-brown color, epidote-group crystal habit (prismatic, often striated), and red streak. Hardness 6.5 and monoclinic crystal system match the broader epidote group.

Distinguish from common epidote (pistachio green vs. red), garnet (isometric crystal system), and thulite (pink zoisite, different color and crystal system). Piemontite's red streak and prismatic habit within metamorphic rock are diagnostic.