igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 on Mohs scale; Color: dark grey to black (weathered to green/brown); Luster: dull to vitreous when wet; Crystal structure: fine-grained/aphanitic; Specific gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- dark grey to black (weathered to green/brown)
- Luster
- dull to vitreous when wet
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Physical properties
Hardness: 6-7 on Mohs scale; Color: dark grey to black (weathered to green/brown); Luster: dull to vitreous when wet; Crystal structure: fine-grained/aphanitic; Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava. Common in oceanic crust and volcanic regions; these specimens are often rounded by wave action on beaches.
Uses & applications
Used primarily in construction as aggregate, for paving stones, and as a raw material for stone wool insulation. Polished stones are used in decoration and aquariums.
Geological facts
Basalt is the most common rock type in the Earth's crust and even makes up the 'maria' or dark plains on the Moon. It is the primary component of the ocean floor.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It is often found as rounded pebbles on volcanic beaches or near tectonic rifts.
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Dark Igneous Rock (Likely Basalt or Diabase)
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.
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