igneous

Basalt (Porphyritic Basalt)

Porphyritic Basalt (Mafic volcanic rock)

Hardness: 6 (Mohs scale); Color: Dark grey to black with distinct darker phenocrysts; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix with visible large crystals (phenocrysts); Cleavage: None/Irregular; Specific gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to black with distinct darker phenocrysts
Luster: Dull to sub-metallic

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The 'porphyritic' texture indicates two stages of cooling: a slow cooling period underground allowing for larger crystals to grow, followed by a rapid eruption and quick cooling of the remaining melt.

Uses & applications

Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Larger slabs are used for architectural cladding and floor tiles. It is occasionally used as 'hot stones' in spa therapy due to its heat retention.

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. The dark spots visible in this specimen are likely pyroxene or olivine phenocrysts.

Field identification & locations

Identify by its heavy weight (density), dark color, and fine-grained texture. To identify this specific specimen in the field, look for the 'salt and pepper' look or clear larger crystals embedded in a dark matte background. Often found in volcanic regions, riverbeds, and coastal areas.

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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)

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

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Magnetite

Magnetite (Fe3O4)

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