metamorphic
Gneissic Granite
Granite Gneiss
Hardness: 6-7 Mohs; Color: Pinkish-tan, cream, and grey with black bands; Luster: Vitreous to dull; Structure: Foliated with visible banding; Cleavage: None (fractures irregularly); Specific gravity: 2.6-2.7.
- Hardness
- 6-7 Mohs
- Color
- Pinkish-tan, cream, and grey with black bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Pinkish-tan, cream, and grey with black bands; Luster: Vitreous to dull; Structure: Foliated with visible banding; Cleavage: None (fractures irregularly); Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the high-grade metamorphism of granite or sedimentary protoliths under intense heat and pressure, typically during mountain-building events (orogeny) millions of years ago.
Uses & applications
Used primarily as crushed stone in construction, road base, and occasionally as structural dimension stone or landscaping material.
Geological facts
The banding in this rock, called foliation, is caused by the segregation of mafic minerals (like biotite) and felsic minerals (like quartz and feldspar) into distinct layers during tectonic compression.
Field identification & locations
Identify by looking for rhythmic dark and light stripes (gneissic banding) and a coarse crystalline texture. Commonly found in eroded mountainous regions or glacial deposits like riverbeds and beaches.
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