igneous

Porphyritic Basalt (with Pyrolusite or Iron-oxide staining)

Porphyritic Basalt (Mafic Extrusive)

Hardness: 5-6 Mohs. Color: Reddish-brown (oxidized) matrix with dark grey/black phenocrysts. Luster: Dull to sub-metallic on inclusions. Crystal structure: Result of rapid cooling. Cleavage: Indistinct.

Hardness: 5-6 Mohs
Luster: Dull to sub-metallic on inclusions

Identified April 6, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium and iron-rich lava at or near the Earth's surface. The distinct 'spots' (phenocrysts) indicate two stages of cooling: a slower stage underground followed by rapid eruption.

Uses & applications

Common basalt is used in road construction, concrete aggregate, and as railroad ballast. This specific weathered specimen is primarily a collector's item or educational sample.

Geological facts

The reddish color is due to the oxidation of iron-rich minerals (chemical weathering or 'rusting' of the rock). Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.

Field identification & locations

Identify in the field by its heavy weight (high density), pitted/vesicular texture, and the presence of larger crystals embedded in a fine-grained groundmass. Look for it in volcanic fields or riverbeds near volcanic activity.

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