igneous
Basalt River Pebble
Basalt (Mafic volcanic rock)
Hardness: 5-6 (Mohs scale); Color: Dark gray to black (weathered to light gray); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark gray to black (weathered to light gray)
- Luster
- Dull/Earthy
Identified More igneous →
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark gray to black (weathered to light gray); Luster: Dull/Earthy; 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 very near the Earth's surface. This specimen has been rounded and smoothed through 'attrition' in a high-energy fluvial (river) or beach environment over thousands of years.
Uses & applications
Commonly used in construction as aggregate, for road base, in high-strength concrete, and for decorative landscaping. Smooth river basalt is also popular in 'hot stone' massage therapy.
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 uniform dark color, heavy weight for its size, and the absence of visible large crystals. Commonly found in areas of past volcanic activity or downstream in riverbeds from volcanic highlands.
