igneous
Granite River Cobble
Phaneritic Felsic Igneous Rock
Hardness: 6-7 Mohs; Color: Mottled salt-and-pepper (tan, black, white); Luster: Dull (weathered) to vitreous (minerals); Crystal structure: Phaneritic (visible crystals of quartz, feldspar, and biotite/hornblende); Rounded due to water erosion.
- Hardness
- 6-7 Mohs
- Color
- Mottled salt-and-pepper (tan, black, white)
- Luster
- Dull (weathered) to vitreous (minerals)
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Physical properties
Hardness: 6-7 Mohs; Color: Mottled salt-and-pepper (tan, black, white); Luster: Dull (weathered) to vitreous (minerals); Crystal structure: Phaneritic (visible crystals of quartz, feldspar, and biotite/hornblende); Rounded due to water erosion.
Formation & geological history
Formed from the slow cooling of silica-rich magma deep underground (plutonic). This specific specimen has been shaped and smoothed by transport in a high-energy water environment, such as a river or glacial stream.
Uses & applications
Used extensively in construction (crushed stone, aggregate), architecture (countertops, monuments), and landscaping. Smooth cobbles like this are popular for decorative garden features.
Geological facts
Granite makes up a large portion of the Earth's continental crust. Because it is very hard and resistant to chemical weathering, it often survives long-distance transport in rivers to become smooth cobbles.
Field identification & locations
Identify by the visible 'speckled' crystals of different colors. It will not be scratched by a steel knife. Commonly found in riverbeds, glacial tills, and mountainous regions worldwide.
