metamorphic
River Cobble (Gneiss/Migmatite)
Gneissic Metamorphic Rock (composed mainly of Quartz, Feldspar, and Biotite)
Hardness: 6-7 (Mohs); Color: Banded white, tan, and dark grey/black; Luster: Dull to vitreous; Crystal Structure: Foliated with visible mineral banding; Cleavage: Poor, typically fractures irregularly along bands; Specific Gravity: 2.6-3.0.
- Hardness
- 6-7 (Mohs)
- Color
- Banded white, tan, and dark grey/black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Banded white, tan, and dark grey/black; Luster: Dull to vitreous; Crystal Structure: Foliated with visible mineral banding; Cleavage: Poor, typically fractures irregularly along bands; Specific Gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under extreme heat and pressure, often deep within the earth's crust. This specimen shows a 'migmatitic' texture where partial melting occurred. Its rounded shape suggests it was weathered and transported by water (fluvial processes).
Uses & applications
Used primarily as decorative garden stone, crushed stone for construction, and as an educational specimen for studying metamorphic textures like foliation and boudinage.
Geological facts
The specimen shows 'boudinage' (sausage-like structures) where hard dark layers have been pulled apart and surrounded by more plastic light-colored minerals. Some gneiss specimens are as old as 4 billion years, representing the oldest crust on Earth.
Field identification & locations
Identify in the field by its distinct alternating bands of dark and light minerals (gneissic banding) and its extreme hardness. It is commonly found in riverbeds, glacial moraines, and mountainous regions like the Appalachians or the Alps.
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