igneous

Basalt

Aphanitic Mafic Igneous Rock

Hardness: 6.0 on Mohs scale; Color: black to dark gray; Luster: dull to sub-vitreous (when polished); Crystal Structure: fine-grained (aphanitic) or glassy; Specific Gravity: 2.8 - 3.0.

Hardness: 6
Color: black to dark gray
Luster: dull to sub-vitreous (when polished)

Identified May 26, 2026More igneous →

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Physical properties

Hardness: 6.0 on Mohs scale; Color: black to dark gray; Luster: dull to sub-vitreous (when polished); Crystal Structure: fine-grained (aphanitic) or glassy; Specific Gravity: 2.8 - 3.0.

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth.

Uses & applications

Basalt is used in construction (crushed stone for road base and railroad ballast), for making stone wool (insulation material), and polished specimens are used as massage stones or decorative pebbles.

Geological facts

Basalt underlies more of Earth's surface than any other rock type. Most of the ocean floor is basalt. It is also found on the Moon, Mars, and Venus.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture with no visible crystals. Often found in volcanic regions or alongside riverbeds as water-worn cobbles.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

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