metamorphic
Greenstone (metamorphosed basalt)
Greenschist / Meta-basalt
Hardness: 6-7 (Mohs scale); Color: Dull green to dark grayish-green; Luster: Dull or earthy to slightly waxy; Crystal structure: Fine-grained, microcrystalline; Cleavage: Poor or absent; Specific gravity: 2.8 - 3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- Dull green to dark grayish-green
- Luster
- Dull or earthy to slightly waxy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dull green to dark grayish-green; Luster: Dull or earthy to slightly waxy; Crystal structure: Fine-grained, microcrystalline; Cleavage: Poor or absent; Specific gravity: 2.8 - 3.0
Formation & geological history
Formed through low-grade metamorphism of mafic igneous rocks (like basalt) under low temperatures and pressures. This process, known as 'greenschist facies' metamorphism, often occurs at convergent plate boundaries or seafloor spreading centers.
Uses & applications
Historically used for stone tools (axes) due to its toughness. Today used as crushed stone for construction, road fill, and sometimes as a decorative building stone or for landscape gardening.
Geological facts
The green color is primarily due to the presence of chlorite, actinolite, or epidote minerals produced during metamorphism. It was highly prized by indigenous cultures globally for making durable 'celt' axes.
Field identification & locations
Identify in the field by its distinct green hue, fine grain, and extreme hardness (it generally will not be scratched by a steel pocket knife). It is commonly found in ancient mountain belts and glacial till. Collectors should look for smooth, river-worn cobbles like this specimen.
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