igneous
Granite Cobble
Phaneritic Felsic Igneous Rock
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (white, gray, black). Luster: Dull to vitreous (sparkling quartz grains). Structure: Holocrystalline/Phaneritic (visible grains). Cleavage: None (fractures irregularly). Specific gravity: 2.63 to 2.75.
- Hardness
- 6-7 on Mohs scale
- Color
- Salt-and-pepper (white, gray, black)
- Luster
- Dull to vitreous (sparkling quartz grains)
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (white, gray, black). Luster: Dull to vitreous (sparkling quartz grains). Structure: Holocrystalline/Phaneritic (visible grains). Cleavage: None (fractures irregularly). Specific gravity: 2.63 to 2.75.
Formation & geological history
Formed from the slow cooling of silica-rich magma deep underground (plutonic). This specific specimen shows signs of water-weathering (rounding), suggesting it was transported by an ancient river or glacial activity, often dating from the Precambrian to Cenozoic eras.
Uses & applications
Used primarily in construction (crushed stone, curbing), dimension stone for countertops and monuments, and as landscaping material. Smaller water-worn cobbles are often used in decorative gardening.
Geological facts
Granite makes up the majority of the Earth's continental crust. Because it contains quartz, it is very resistant to chemical weathering, which is why boulders like this can persist in riverbeds for thousands of years.
Field identification & locations
Identify in the field by its 'salt and pepper' appearance and visible interlocking crystals of quartz, feldspar, and mica. It cannot be scratched by a steel knife. Commonly found in mountainous regions or glacial till deposits.
