metamorphic
Greenstone (Epidote/Chlorite-rich Metabasalt)
Greenschist / Epidosite (Ca2(Al,Fe)3(SiO4)3(OH))
Hardness: 6-7 (Mohs scale); Color: Pale to dark forest green, often with mottled brown or gray; Luster: Dull to vitreous; Crystal Structure: Monoclinic (usually massive or microcrystalline); Cleavage: Perfect in one direction (often obscured in massive forms); Specific Gravity: 3.2-3.5.
- Hardness
- 6-7 (Mohs scale)
- Color
- Pale to dark forest green, often with mottled brown or gray
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Pale to dark forest green, often with mottled brown or gray; Luster: Dull to vitreous; Crystal Structure: Monoclinic (usually massive or microcrystalline); Cleavage: Perfect in one direction (often obscured in massive forms); Specific Gravity: 3.2-3.5.
Formation & geological history
Formed through low-grade regional metamorphism of mafic igneous rocks (basalts and gabbros) under high pressure and low temperatures (200-400°C), typically during the Paleoproterozoic to Archean eras. The green color is derived from chlorite, actinolite, and epidote minerals replacing original dark minerals.
Uses & applications
Historically used for making stone tools (celts and axes) due to its toughness; currently used as crushed stone for construction, architectural veneers, and occasionally polished for lapidary use or as a minor gemstone (Unakite variety).
Geological facts
Greenstone belts are orogenic belts of highly metamorphosed rock that are critical to our understanding of the Earth’s early crustal evolution. They are often associated with major world-class gold deposits.
Field identification & locations
Identify in the field by its characteristic 'dull' green color that does not scratch easily with steel. Found frequently in the Canadian Shield, Western Australia, and the Appalachian Mountains. Collectors look for pieces with distinct epidote veining.
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