igneous

Porphyritic Basalt

Basalt with plagioclase phenocrysts

Hardness: 6-7 (Mohs scale), Color: Dark grey to black groundmass with light-colored (white/cream) phenocrysts, Luster: Dull to sub-vitreous, Structure: Aphanitic matrix with visible crystals (porphyritic texture), Specific Gravity: 2.8-3.0

Identified May 16, 2026More igneous →

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Physical properties

Formation & geological history

Formed via rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface, where larger crystals (phenocrysts) began growing underground before the final eruption. Common in volcanic regions and ocean floor crust.

Uses & applications

Primarily used in construction as crushed stone, road base, or concrete aggregate. High-quality specimens are used in landscaping or as educational geological samples.

Geological facts

The light spots are called phenocrysts, usually consisting of plagioclase feldspar. This texture tells a story of two-stage cooling: slow cooling deep underground followed by rapid cooling during an eruption.

Field identification & locations

Identify by the 'salt and pepper' look where large distinct light crystals are embedded in a fine-grained dark background. Often found on beaches or in glacial till in regions with volcanic history.

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