igneous
Basalt (Polished Pebble)
Basalt (Mafic volcanic rock)
Hardness: 6 (Mohs scale); Color: Black to dark gray; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Black to dark gray
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 (Mohs scale); Color: Black to dark gray; Luster: Dull to sub-vitreous; 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 near the Earth's surface. This specimen appears water-worn or mechanically tumbled to achieve its sleek, rounded shape.
Uses & applications
Commonly used as river stones for landscaping, massage stones (due to heat retention), and as aggregate in construction/road building.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large parts of the Moon's surface, the lunar maria, are also composed of basalt.
Field identification & locations
Identify by its heavy weight relative to size (high density) and dark, uniform color. Found globally in volcanic regions and often as smooth pebbles in riverbeds or on beaches near volcanic coasts.
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Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.
Igneous
Magnetite
Iron(II,III) oxide (Fe3O4)
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Blue Apatite
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Magnetite
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