igneous
River Cobble (likely Basalt or Mafic Igneous Rock)
Extrusive Mafic Igneous Rock (e.g., Basalt)
Hardness: 5-7 Mohs; Color: Dark grey to blackish-brown with visible surface weathering; Luster: Dull/Earthy (weathered) to Sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8 - 3.0
- Hardness
- 5-7 Mohs
- Color
- Dark grey to blackish-brown with visible surface weathering
- Luster
- Dull/Earthy (weathered) to Sub-metallic
Identified More igneous →
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Physical properties
Hardness: 5-7 Mohs; Color: Dark grey to blackish-brown with visible surface weathering; Luster: Dull/Earthy (weathered) to Sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. This specimen has been rounded and smoothed by fluvial (river) erosion over thousands of years.
Uses & applications
Commonly used in construction as crushed stone, aggregate for concrete, and road base. Also used in aquarium landscaping and rock gardening.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (high density), and fine-grained texture. Look for pits or vesicles if it is volcanic, though this specimen is water-worn. Found globally in volcanic regions and riverbeds.
