igneous
River Cobble (Basalt or Greywacke)
Extrusive Mafic Igneous Rock (likely Basalt)
Hardness: 5-7. Color: Grey to dark grey. Luster: Dull/Earthy. Structure: Fine-grained (aphanitic) to microcrystalline. Shape: Ellipsoidal smoothed by hydraulic action.
- Hardness
- 5-7
- Color
- Grey to dark grey
- Luster
- Dull/Earthy
Identified More igneous →
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Physical properties
Hardness: 5-7. Color: Grey to dark grey. Luster: Dull/Earthy. Structure: Fine-grained (aphanitic) to microcrystalline. Shape: Ellipsoidal smoothed by hydraulic action.
Formation & geological history
Formed from rapid cooling of magnesium-rich and iron-rich lava. It has been subsequently eroded and shaped by high-energy water environments like rivers or coastal beaches over thousands of years.
Uses & applications
Commonly used in landscaping, decorative masonry, pebble mosaics, and as a natural stress relief tool (worry stones). Industrially, crushed basalt is used for road base and concrete aggregate.
Geological facts
This specimen is a classic example of textural maturity; its rounded shape indicates it has traveled a long distance from its source through water transport. Basalt is the most common rock type in Earth\"s crust, making up most of the ocean floor.
Field identification & locations
Identify by its uniform grey color, heavy weight (high specific gravity), and smooth, water-worn texture. Commonly found in riverbeds, glacial deposits, and along rocky shorelines.
