igneous

Basalt (Porphyritic)

Porphyritic Mafic Extrusive (Basalt)

Hardness: 6 (Mohs scale); Color: Dark grey to charcoal grey; Luster: Dull/Earthy (matrix) to Sub-vitreous (phenocrysts); Crystal structure: Aphanitic (fine-grained) matrix with visible phenocrysts; Cleavage: Poorly developed; Specific Gravity: 2.8-3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to charcoal grey
Luster: Dull/Earthy (matrix) to Sub-vitreous (phenocrysts)

Identified March 20, 2026More igneous →

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Physical properties

Formation & geological history

Basalt is formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Porphyritic texture indicates a two-stage cooling process: a slow cooling phase deep underground forming larger crystals, followed by an eruptive phase that cools the remaining melt quickly.

Uses & applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Larger slabs are used for floor tiles, paving stones, and monuments.

Geological facts

Basalt is the most common rock in the Earth's crust and makes up most of the ocean floor. It is also found on other planetary bodies like Mars and the Moon (the dark 'seas' or Maria are basaltic).

Field identification & locations

Identify in the field by its high density, dark grey color, and the presence of small, dark, or greenish crystals (phenocrysts) embedded in a fine-grained background. It is ubiquitous in volcanic regions such as the Pacific Northwest of the USA, Iceland, and Hawaii.

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