Igneous

Basalt

Mafic Extrusive Igneous Rock

Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of plagioclase or olivine; Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark grey to black, weathering to brown
Luster: Dull/Matte

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic lava) exposed at or very near the surface of a terrestrial planet or moon. Ages vary widely from billions of years old to recent volcanic flows.

Uses & applications

Extensively used in construction as aggregate (crushed stone) for road base, railroad ballast, and concrete. It is also used in high-temp insulation (mineral wool) and as a building stone or 'paver' in landscape architecture.

Geological facts

Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor. It also makes up the 'maria' (seas) on the Moon, which are actually giant ancient basaltic lava plains.

Field identification & locations

Identify in the field by its dark color, heavy weight (relative to size), and very fine grains that are hard to see without a lens. Often found in volcanic regions, oceanic islands (like Hawaii), and mid-ocean ridges. Collectors look for vesicles (gas bubbles) filled with secondary minerals like zeolites or amethysts.

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