igneous
Granite
Phaneritic plutonic igneous rock (SiO2 range 65-75%)
Hardness: 6-7 Mohs; Color: Gray, pinkish-tan, and white with dark flecks; Luster: Dull (weathered) to vitreous (crystals); Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.63 to 2.75
- Hardness
- 6-7 Mohs
- Color
- Gray, pinkish-tan, and white with dark flecks
- Luster
- Dull (weathered) to vitreous (crystals)
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, pinkish-tan, and white with dark flecks; Luster: Dull (weathered) to vitreous (crystals); Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.63 to 2.75
Formation & geological history
Formed from the slow cooling of silica-rich magma deep underground (plutonic). This sample appears to be a water-worn river cobble or glacial erratic, likely dating from the Precambrian to Cenozoic depending on location.
Uses & applications
Used extensively in construction (aggregate), architectural facades, countertops, and paving. Highly valued for durability and resistance to weathering.
Geological facts
Granite makes up a large portion of the Earth's continental crust. Mount Rushmore is carved into a massive granite batholith in South Dakota.
Field identification & locations
Identify by its interlocking salt-and-pepper appearance and visible individual crystals of quartz, feldspar, and mica. Commonly found in mountainous regions or as rounded stones in stream beds.
