metamorphic
Greenstone / Metagraywacke
Metamorphosed basaltic or sedimentary rock (Complex Silicate)
Hardness: 6-7 (Mohs); Color: Dull green, olive, grayish-green; Luster: Dull to waxy; Texture: Fine-grained, dense; Cleavage: None (conchoidal fracture)
- Hardness
- 6-7 (Mohs)
- Color
- Dull green, olive, grayish-green
- Luster
- Dull to waxy
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Physical properties
Hardness: 6-7 (Mohs); Color: Dull green, olive, grayish-green; Luster: Dull to waxy; Texture: Fine-grained, dense; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the low-grade metamorphism of mafic igneous rocks (like basalt) or graywacke sandstone under low temperature and high pressure conditions, typical of subduction zones.
Uses & applications
Used historically for stone tools (axes), currently used as crushed stone for construction, road ballast, and occasionally for lapidary work or aquarium decor.
Geological facts
The green color is primarily due to the presence of chlorite, epidote, or actinolite minerals. Greenstones are often found in ancient oceanic crust fragments called ophiolites.
Field identification & locations
Identify in the field by its distinct greenish hue, extreme toughness, and lack of visible crystals. Commonly found in riverbeds in orogenic (mountain-building) belts like the Pacific Northwest or Appalachians.
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