igneous
River Cobble (Weathered Granitoid)
Granodiorite / Quartz Diorite
Hardness: 6-7 Mohs; Color: Mottled gray, white, and salt-and-pepper black; Luster: Dull/Earthy due to weathering; Structure: Phaneritic (medium-grained crystalline); Primary minerals: Quartz, Plagioclase feldspar, and Biotite or Hornblende.
- Hardness
- 6-7 Mohs
- Color
- Mottled gray, white, and salt-and-pepper black
- Luster
- Dull/Earthy due to weathering
Identified More igneous →
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Physical properties
Hardness: 6-7 Mohs; Color: Mottled gray, white, and salt-and-pepper black; Luster: Dull/Earthy due to weathering; Structure: Phaneritic (medium-grained crystalline); Primary minerals: Quartz, Plagioclase feldspar, and Biotite or Hornblende.
Formation & geological history
Formed from the slow cooling of magma deep underground, followed by uplift and erosion. This specific specimen shows significant rounding and smoothing from transport in a high-energy water environment like a river or coastal region.
Uses & applications
Common river stones are used in landscaping, construction aggregate, and as decorative garden stones. If particularly high in quartz, they may be crushed for industrial use.
Geological facts
This rock exhibits a 'rinded' surface, where chemical weathering has slightly altered the exterior minerals compared to the fresh rock inside. It is a very common component of continental crust.
Field identification & locations
Identify by its interlocking crystalline texture and lack of layering (foliation). To see the true mineral composition, one would need to see a fresh break. Found globally in creek beds and glacial till.
