igneous
River Rock (Granite Cobble)
Granite (Felsic Plutonic Rock)
Hardness: 6-7 on Mohs scale. Color: Mottled tan, grey, and black. Luster: Dull to slightly sparkling (due to mica/quartz). Crystal Structure: Phaneritic (medium-grained crystalline). Cleavage: None for the rock itself; specific gravity: 2.63 to 2.75.
- Hardness
- 6-7 on Mohs scale
- Color
- Mottled tan, grey, and black
- Luster
- Dull to slightly sparkling (due to mica/quartz)
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Mottled tan, grey, and black. Luster: Dull to slightly sparkling (due to mica/quartz). Crystal Structure: Phaneritic (medium-grained crystalline). Cleavage: None for the rock itself; specific gravity: 2.63 to 2.75.
Formation & geological history
Formed from the slow cooling of silica-rich magma deep underground, likely during the Precambrian or Paleozoic eras. It has since been unearthed and rounded through water erosion in a river or glacial environment.
Uses & applications
Commonly used in landscaping, decorative ground cover, aquarium decor, and as garden stones. In bulk, crushed granite is used for construction and road bases.
Geological facts
Granite is one of the oldest and most abundant rocks on Earth's continental crust. The rounded shape indicates it has traveled long distances in water, which smoothed away its original jagged edges.
Field identification & locations
Identify by its 'salt and pepper' appearance and visible interlocking crystals of quartz (clear/grey), feldspar (tan/pinkish), and biotite or hornblende (black). Frequently found in glacial till, riverbeds, and mountainous regions.
