metamorphic
Metagabbro / Epidote Gneiss
Epidote bearing Hornblende Gneiss
Hardness: 6-7 (Mohs scale); Color: Mottled green, grey, and black with visible banding; Luster: Dull to vitreous when wet; Structure: Foliated/Gneissose; Cleavage: None (rock mass); Specific Gravity: 2.9-3.3
- Hardness
- 6-7 (Mohs scale)
- Color
- Mottled green, grey, and black with visible banding
- Luster
- Dull to vitreous when wet
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Mottled green, grey, and black with visible banding; Luster: Dull to vitreous when wet; Structure: Foliated/Gneissose; Cleavage: None (rock mass); Specific Gravity: 2.9-3.3
Formation & geological history
Formed through the regional metamorphism of mafic igneous rocks (like gabbro or basalt). Under high pressure and temperature, minerals like pyroxene transform into hornblende and epidote, creating the characteristic green and black bands.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and occasionally as decorative stone or 'riprap' for erosion control.
Geological facts
The distinctive pistachio-green color is typically caused by the presence of the mineral epidote. These rocks often represent bits of ancient oceanic crust that have been caught up in continental mountain-building events.
Field identification & locations
Identify in the field by its distinct green/dark color banding and high density. Commonly found in metamorphic terranes such as the Appalachian Mountains or the Canadian Shield. Collectors look for well-defined bands.
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