igneous
River Pebble (Basalt)
Basalt (Aphanitic Mafic Igneous Rock)
Hardness: 6 Mohs; Color: dark grey to blackish-brown; Luster: dull/earthy; Crystal structure: fine-grained (microcrystalline); Cleavage: none; Specific gravity: 2.8-3.0.
- Hardness
- 6 Mohs
- Color
- dark grey to blackish-brown
- Luster
- dull/earthy
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Physical properties
Hardness: 6 Mohs; Color: dark grey to blackish-brown; Luster: dull/earthy; Crystal structure: fine-grained (microcrystalline); Cleavage: none; Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The rounded shape is the result of mechanical weathering and abrasion in a high-energy aquatic environment, such as a river or shoreline.
Uses & applications
Used primarily in construction as aggregate for concrete or asphalt, road base, and decorative landscaping. Small rounded stones are often used for drainage or decorative garden mulch.
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 by its dark color, heavy weight for its size, and lack of visible crystals. Commonly found in riverbeds, glacial deposits, and along volcanic coastlines.
