Epidote

Hardness: 6-7 on Mohs scale. Color: Typically pistachio-green, yellowish-green, brownish-green, or black; sometimes dark green to black in thick crystals. Luster: Vitreous to resinous. Crystal structure: Monoclinic, often forming prismatic crystals with vertical striations, or massive and granular aggregates. Cleavage: Good in one direction. Specific Gravity: 3.3-3.5.

Epidote is a common metamorphic mineral that forms under moderate-to-high pressure and temperature conditions. It is frequently found in schists, gneisses, skarns, and altered igneous rocks (like greenschists) where plagioclase feldspar has been altered. It can also occur in hydrothermal veins. Its formation is associated with regional and contact metamorphism. Geological Age: Varies widely, depending on the metamorphic event, from Precambrian to recent.

While individual crystals can be cut into attractive gemstones (especially the transparent, vibrant green varieties, sometimes called 'pistacite'), its primary use is as an indicator mineral in geological studies, helping to interpret metamorphic conditions and rock histories. It's not a major industrial mineral.

The name 'epidote' comes from the Greek word 'epidosis' meaning 'addition,' referring to the fact that one side of the prism is longer than the other. It is a member of the epidote group of minerals, which are nesosilicates. Epidote often replaces other minerals such as plagioclase feldspar, hornblende, and garnet in metamorphic rocks.

In the field, epidote can be identified by its distinctive pistachio-green color (though it can vary), vitreous luster, and typically prismatic crystal habit. It often occurs with quartz, feldspar, hornblende, and garnet. Look for it in metamorphic terrains, skarns, and altered igneous rocks. Common locations include the Alps (Europe), Baja California (Mexico), Alaska (USA), and various localities in Norway and Pakistan. Collectors value well-formed, transparent crystals, especially those with good color.