igneous
Granite (River Cobble)
Granite (rich in feldspar, quartz, and biotite)
Porphyritic or phaneritic texture; Hardness: 6-7 on Mohs scale; Color: mottled brown, tan, and black due to iron staining; Luster: dull to vitreous; Structure: crystalline aggregate; Specific gravity: 2.63-2.75.
- Hardness
- 6-7 on Mohs scale
- Color
- mottled brown, tan, and black due to iron staining
- Luster
- dull to vitreous
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Physical properties
Porphyritic or phaneritic texture; Hardness: 6-7 on Mohs scale; Color: mottled brown, tan, and black due to iron staining; Luster: dull to vitreous; Structure: crystalline aggregate; Specific gravity: 2.63-2.75.
Formation & geological history
Formed from the slow cooling of silica-rich magma deep underground (plutonic). This specific specimen is a rounded cobble, shaped by water erosion in a river or glacial environment.
Uses & applications
Used extensively in construction (curbstones, floor tiles), kitchen countertops, and as crushed stone for road base. Rounded pebbles like this are often used for decorative landscaping.
Geological facts
Granite makes up a large portion of the Earth's continental crust. The reddish-brown tint in this specimen suggests oxidation of iron-bearing minerals or high potassium feldspar content.
Field identification & locations
Identify by its 'salt and pepper' crystalline appearance and extreme hardness (cannot be scratched by a steel knife). Commonly found in mountainous regions or riverbeds downstream from igneous outcrops.
