igneous
Basalt River Cobble
Basalt (Composition primarily Plagioclase Feldspar and Pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull or earthy
Identified More igneous →
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull or 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 specific specimen has been shaped and smoothed into an ovoid 'cobble' through long-term river transport (fluvial erosion).
Uses & applications
Used widely in the construction industry as aggregate (crushed stone), for paving, and as 'river rock' for landscaping. High-quality smooth stones are often used in massage therapy (hot stone massage).
Geological facts
Basalt is the most common volcanic rock on Earth and makes up the majority 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 (density), and lack of visible large crystal grains without magnification. Commonly found along beaches, riverbeds, and in volcanic regions.
