igneous
River Cobble (likely Basalt or Chert)
Basalt (Mafic volcanic rock)
Hardness: 6-7 Mohs; Color: Dark grey to black; Luster: Dull/Sub-metallic; Structure: Fine-grained (Aphanitic); Shape: Smooth, rounded water-worn cobble.
- Hardness
- 6-7 Mohs
- Color
- Dark grey to black
- Luster
- Dull/Sub-metallic
Identified More igneous →
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to black; Luster: Dull/Sub-metallic; Structure: Fine-grained (Aphanitic); Shape: Smooth, rounded water-worn cobble.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. This specific specimen has been shaped by fluvial erosion (river transport) over thousands of years.
Uses & applications
Commonly used in landscaping, road construction as crushed aggregate, and as 'river rocks' for decorative garden features.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. When rounded by water, they are often referred to as 'worry stones'.
Field identification & locations
Identify by its dark color, heavy weight for its size, and lack of visible crystals. Commonly found in riverbeds, glacial deposits, and coastal beaches.
