metamorphic
Greenstone (Greenschist facies)
Greenschist (composed primarily of Chlorite, Actinolite, and Epidote)
Hardness: 3.5-5 (variable based on mineral mix); Color: Dull green, forest green, to grayish-green; Luster: Pearly to dull; Structure: Foliated or massive; Cleavage: Good in Schistose varieties; Specific Gravity: 2.7-3.0.
- Hardness
- 3
- Color
- Dull green, forest green, to grayish-green
- Luster
- Pearly to dull
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Physical properties
Hardness: 3.5-5 (variable based on mineral mix); Color: Dull green, forest green, to grayish-green; Luster: Pearly to dull; Structure: Foliated or massive; Cleavage: Good in Schistose varieties; Specific Gravity: 2.7-3.0.
Formation & geological history
Formed through low-grade regional metamorphism (greenschist facies) of mafic igneous rocks like basalt or gabbro. This occurs at temperatures of 300 to 450 degrees Celsius and depths of 5 to 20 kilometers.
Uses & applications
Used historically for tools and weapons (e.g., adzes); currently used as crushed stone in construction, decorative garden stone, and occasionally for carvings or cabochons if the texture is fine enough.
Geological facts
Greenstone belts represent some of the oldest known rocks on Earth, dating back to the Archean and Proterozoic eons. The green color is derived from chlorite, which is a common alteration product of dark silicate minerals.
Field identification & locations
Identify in the field by its characteristic green hue and earthy, non-metallic appearance. It is often found in mountainous regions or ancient shield areas like the Canadian Shield or the Appalachian Mountains. Collectors should look for signs of 'slaty' or 'schistose' layering.
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