igneous

Basalt with Plagioclase Phenocrysts (Porphyritic Basalt)

Porphyritic Basalt (primarily SiO2, Al2O3, FeO, MgO, CaO)

Hardness: 6-7 on Mohs scale. Color: Dark grey to black groundmass with light-colored (tan or white) rectangular inclusions. Luster: Dull to sub-vitreous. Texture: Porphyritic (large crystals in a fine-grained matrix). Specific Gravity: 2.8-3.0.

Hardness: 6-7 on Mohs scale
Luster: Dull to sub-vitreous

Identified March 12, 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 larger crystals (phenocrysts) formed slowly underground before the final eruption. Common in volcanic fields and oceanic crust.

Uses & applications

Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally durable specimens are used for garden landscaping or decorative 'river rocks.'

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 in abundance on the Moon and Mars. The contrasting light crystals are often plagioclase feldspar.

Field identification & locations

Identify by looking for a dense, dark, heavy rock with distinct rectangular or lath-shaped crystals embedded within it. Commonly found in areas with historic volcanic activity or in glacial till and riverbeds where they have been transported and rounded by water.

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