igneous

Porphyritic Basalt

Porphyritic Basalt (with Plagioclase and Augite phenocrysts)

Hardness: 6.0; Color: Dark grey to greenish-black matrix with lighter phenocrysts; Luster: Dull/Sub-vitreous; Crystal Structure: Aphanitic matrix with visible crystals; Specific Gravity: 2.8-3.0

Hardness: 6
Color: Dark grey to greenish-black matrix with lighter phenocrysts
Luster: Dull/Sub-vitreous

Identified May 25, 2026More igneous →

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Physical properties

Hardness: 6.0; Color: Dark grey to greenish-black matrix with lighter phenocrysts; Luster: Dull/Sub-vitreous; Crystal Structure: Aphanitic matrix with visible crystals; Specific Gravity: 2.8-3.0

Formation & geological history

Formed from the rapid cooling of basaltic lava on the surface, where larger crystals (phenocrysts) grew slowly at depth before the final eruption. Common in volcanic island arcs and seafloor spreading centers.

Uses & applications

Used primarily as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally durable for construction.

Geological facts

The 'two-stage' cooling process creates the distinctive 'dalmatian' or speckled look. Basalt is the most common rock type in the Earth's crust and even makes up much of the Moon's lunar mare.

Field identification & locations

Identify by looking for a fine-grained dark background stone containing distinct, larger individual mineral crystals. Frequently found on beaches or in riverbeds near volcanic regions.

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral