Igneous

Basalt

Mafic volcanic rock (Primarily Plagioclase Feldspar and Pyroxene)

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark gray to black
Luster: Dull to sub-metallic

Identified March 18, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); 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. Common in oceanic crust and continental flood basalts.

Uses & applications

Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate; also used for high-durability floor tiles and garden landscaping.

Geological facts

Basalt is the most common rock type in the Earth's crust and also makes up most of the surface of the Moon and Mars.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and lack of visible large crystals. It often has sharp edges when freshly broken (conchoidal fracture). Common in volcanic regions.

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral