igneous
Granite Cobble
Granitoid (typically containing Quartz SiO2, Feldspar KAlSi3O8, and Mica minerals)
Hardness: 6-7 on Mohs scale; Color: Pinkish-tan with visible dark mineral flecks; Luster: Dull (weathered surface); Structure: Phaneritic (coarse-grained) crystalline texture; Cleavage: None (fractures irregularly); Specific Gravity: 2.63 to 2.75.
- Hardness
- 6-7 on Mohs scale
- Color
- Pinkish-tan with visible dark mineral flecks
- Luster
- Dull (weathered surface)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-tan with visible dark mineral flecks; Luster: Dull (weathered surface); Structure: Phaneritic (coarse-grained) crystalline texture; Cleavage: None (fractures irregularly); Specific Gravity: 2.63 to 2.75.
Formation & geological history
Formed from the slow subterranean cooling of silica-rich magma (intrusive igneous). This specific specimen has been rounded by fluvial (river) or glacial transport, a process that can take thousands of years. Its age can range from hundreds of millions to billions of years depending on the source pluton.
Uses & applications
Used widely in construction as crushed stone or aggregate. High-quality samples are used for countertops, curbing, and monuments. Smooth cobbles like this are popular in landscaping and decorative masonry.
Geological facts
Granite makes up a large portion of the Earth's continental crust. Because it is so hard and resistant to erosion, rounded granite cobbles are often found far from their original source, transported by ancient glaciers or floodwaters.
Field identification & locations
Identify in the field by its 'salt and pepper' appearance of interlocking crystals and its extreme hardness (it will scratch glass but not be scratched by a steel blade). Commonly found in riverbeds, glacial moraines, and rocky coastlines.
