igneous
Basalt (Weathered/River worn)
Mafic Volcanic Rock (Primarily Plagioclase Feldspar and Pyroxene)
Hardness: 6-7 (Mohs); Color: Dark grey, black, or dark greenish-brown; Luster: Dull to greasy (when wet); Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
- Hardness
- 6-7 (Mohs)
- Color
- Dark grey, black, or dark greenish-brown
- Luster
- Dull to greasy (when wet)
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey, black, or dark greenish-brown; Luster: Dull to greasy (when wet); 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 appears to be a river-worn cobble, rounded by water erosion and potentially reflecting moss or mineral staining.
Uses & applications
Used extensively in construction as aggregate for concrete and asphalt, as road base, and as railroad ballast. Some varieties are used in decorative stone landscaping or as 'hot stones' in massage therapy.
Geological facts
Basalt is the most common rock type on Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.
Field identification & locations
Identify in the field by its heavy weight, dark color, and lack of visible crystals to the naked eye. This specimen's rounded shape indicates it was transported by water. Commonly found in volcanic regions and riverbeds down-stream from volcanic outcrops.
