igneous

Basalt (with Plagioclase Phenocrysts)

Porphyritic Basalt

Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with white needle-like crystals, Luster: Dull or earthy matrix with vitreous inclusions, Crystal structure: Fine-grained aphanitic matrix with larger phenocrysts, Specific Gravity: 2.8–3.0

Identified April 5, 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 very near the Earth's surface. The white flecks (phenocrysts) began cooling slowly underground before the final eruption. Common throughout various geological eras.

Uses & applications

Commonly used as crushed stone for road base, concrete aggregate, and railroad ballast. Some porphyritic varieties are used as decorative garden stones or architectural veneers.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The light-colored needle-like crystals in this specimen are likely plagioclase feldspar.

Field identification & locations

Identify by its dark, heavy, and fine-grained texture with distinct lath-shaped white crystals. It is ubiquitous in volcanic regions such as the Pacific Northwest, Hawaii, or Iceland.

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