igneous
Granodiorite (River Cobble)
Granodiorite (contains Quartz, Plagioclase Feldspar, Biotite Mica)
Hardness: 6-7 Mohs; Color: Mottled grey, green-yellow (due to oxidation/weathering), and black; Luster: Dull/earthy (weathered surface); Texture: Phaneritic (visible grains); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 Mohs
- Luster
- Dull/earthy (weathered surface)
Identified More igneous →
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Physical properties
Hardness: 6-7 Mohs; Color: Mottled grey, green-yellow (due to oxidation/weathering), and black; Luster: Dull/earthy (weathered surface); Texture: Phaneritic (visible grains); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the slow cooling of silica-rich magma beneath the Earth's surface (plutonic). This specific specimen appears to be a river cobble, shaped and rounded by water transport over thousands of years.
Uses & applications
Commonly used in the construction industry as crushed stone for road base, concrete aggregate, and sometimes polished as curbing or countertops in architectural applications.
Geological facts
Granodiorite is intermediate between granite and diorite. It is the rock type that makes up the famous Rosetta Stone and much of the Sierra Nevada batholith.
Field identification & locations
Identify by looking for a 'salt and pepper' appearance of minerals like quartz and mica, and a resistance to scratching with steel. Common in riverbeds and mountainous glacial runoff areas.
