metamorphic
Greenstone / Graywacke
Metagraywacke (Chlorite-grade schistose rock)
Hardness: 6-7 Mohs; Color: Dull grayish-green with salt-and-pepper texture; Luster: Dull to earthy; Crystal Structure: Microcrystalline with occasional visible quartz grains; Cleavage: None (conchoidal to irregular fracture); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Color
- Dull grayish-green with salt-and-pepper texture
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Dull grayish-green with salt-and-pepper texture; Luster: Dull to earthy; Crystal Structure: Microcrystalline with occasional visible quartz grains; Cleavage: None (conchoidal to irregular fracture); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed from the low-grade metamorphism of mafic igneous rocks or sedimentary graywacke. This transformation typically occurs in subduction zones or convergent plate boundaries under moderate pressure and temperature, often during the Paleozoic or Mesozoic eras.
Uses & applications
Commonly used as crushed stone for road construction, railroad ballast, and riprap for erosion control. Occasionally used for decorative landscaping or as garden cobbles.
Geological facts
The green color is primarily due to the presence of chlorite, epidote, or actinolite minerals. In ancient times, varieties like 'Green Porphyry' were highly prized by Romans for sculptures and architectural elements.
Field identification & locations
Identify in the field by its distinct greenish-gray hue, hardness (it will not be easily scratched by a steel knife), and its rounded, water-worn shape if found in riverbeds or coastal areas. Common in orogenic belts like the Appalachians or the Scottish Highlands.
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