igneous
Granite Cobble
Granite (Phaneritic Quartz-Feldspar specimen)
Hardness: 6-7 (Mohs scale); Color: Mottled brown, tan, and black; Lustre: Dull to vitreous; Crystal structure: Coarse-grained (phaneritic); Cleavage: Indistinct; Specific gravity: approx. 2.6 – 2.7.
- Hardness
- 6-7 (Mohs scale)
- Color
- Mottled brown, tan, and black
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Mottled brown, tan, and black; Lustre: Dull to vitreous; Crystal structure: Coarse-grained (phaneritic); Cleavage: Indistinct; Specific gravity: approx. 2.6 – 2.7.
Formation & geological history
Formed from the slow cooling of silica-rich magma deep underground (plutonic). This specific specimen shows a rounded shape (cobble), indicating it was later eroded and transported by water or glacial activity.
Uses & applications
Used extensively in construction for countertops, floor tiles, and gravel. Polished pebbles are used in landscaping and aquarium decor.
Geological facts
Granite makes up the majority of the Earth's continental crust. It is primarily composed of quartz, fieldspar, and micas (biotite/muscovite). This specific specimen's brownish hue is likely due to iron oxide staining from weathering.
Field identification & locations
Identify by its interlocking 'salt and pepper' crystalline texture and extreme hardness (cannot be scratched by a steel knife). Commonly found in riverbeds, glacial moraines, and mountain ranges.
