igneous
Basalt River Cobble
Basalt (Mafic Igneous Rock)
Hardness: 6 (Mohs), Color: Dark grey to black with greenish flecks (likely olivine), Luster: Dull to sub-vitreous when wet, Structure: Microcrystalline/aphanitic, Specific Gravity: 2.8–3.0.
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Physical properties
Hardness: 6 (Mohs), Color: Dark grey to black with greenish flecks (likely olivine), Luster: Dull to sub-vitreous when wet, Structure: Microcrystalline/aphanitic, 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 rounded by mechanical weathering in a river or glacial environment.
Uses & applications
Commonly used in construction as crushed aggregate, for paving, and in 'hot stone' massage therapy due to high heat retention. Rounded specimens are popular for landscaping and garden aesthetics.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most 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 crystals. Look for basalt in volcanic regions or in riverbeds downstream from volcanic mountain ranges.
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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.
Igneous