igneous

Basalt

Hardness: 6-7 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8-3.0; Cleavage: None.

Hardness: 6-7 (Mohs)
Color: Dark grey to black
Luster: Dull to sub-metallic

Identified May 9, 2026More igneous →

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Physical properties

Hardness: 6-7 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8-3.0; Cleavage: None.

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.

Uses & applications

Commonly used in construction as aggregate (crushed stone) for road base, railroad ballast, and concrete; also used in stone carvings and as thermal insulation in the form of rock wool.

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, Mars, and Venus.

Field identification & locations

Identify in the field by its dark color, heavy weight (high density), and fine-grained texture. Often found in volcanic regions, oceanic islands, and mid-ocean ridges. Collectors look for vesicular basalt (with gas bubbles) or basalt containing olivine crystals.

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