igneous
Basalt Cobble
Aphanitic Mafic Igneous Rock (Basalt)
Hardness: 6.0 on Mohs scale; Color: Dark grey to black or brownish-black; Luster: Dull to sub-metallic (shiny when wet); Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0
- Hardness
- 6
- Color
- Dark grey to black or brownish-black
- Luster
- Dull to sub-metallic (shiny when wet)
Identified More igneous →
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Physical properties
Hardness: 6.0 on Mohs scale; Color: Dark grey to black or brownish-black; Luster: Dull to sub-metallic (shiny 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. Extrusive volcanic origin, commonly found in oceanic crust and lava flows.
Uses & applications
Extensively used in construction for aggregate, road base, and railroad ballast. Smooth river-worn specimens are popular for decorative landscaping and hot stone massage therapy.
Geological facts
Basalt is the most common rock type on Earth's surface and also makes up much of the surface of the Moon and Mars. It is the primary rock of the oceanic crust.
Field identification & locations
Identified by its heavy weight (high density), dark matte color, and lack of visible crystals to the naked eye. Often found on beaches or in riverbeds as rounded 'river stones' or cobbles.
