igneous

Porphyritic Basalt

Basalt (with plagioclase phenocrysts)

Hardness: 6 (Mohs); Color: Dark brown/grey matrix with white inclusions; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine grain) with visible phenocrysts; Cleavage: None visible.

Hardness: 6 (Mohs)
Color: Dark brown/grey matrix with white inclusions
Luster: Dull to sub-vitreous

Identified May 16, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs); Color: Dark brown/grey matrix with white inclusions; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine grain) with visible phenocrysts; Cleavage: None visible.

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the surface. The white spots represent crystals (phenocrysts) that began cooling underground before the final eruption.

Uses & applications

Used primarily in construction (crushed stone), road base, and occasionally as pocket stones or river rocks for landscaping.

Geological facts

Basalt is common on Earth but also makes up most of the lunar mare on the Moon and the surface of Mars. It is the most common volcanic rock on Earth.

Field identification & locations

Identify by the contrast between a fine-grained dark matrix and larger crystalline 'spots'. Found commonly in volcanic regions, riverbeds, and glacial till.

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