Igneous

Porphyritic Basalt or Andesite

Porphyritic Basalt/Andesite

Dark gray to brown groundmass with lighter rounded or subangular phenocrysts (likely plagioclase feldspar). Hardness is generally 5.5 to 6 on the Mohs scale. Dull luster, fine-grained groundmass with macroscopic crystals.

Identified May 31, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of magma or lava. The two distinct crystal sizes indicate a two-stage cooling process: a slow cooling deep within the earth forming the larger phenocrysts, followed by an eruption or rapid cooling forming the finer-grained matrix.

Uses & applications

Often used as aggregate in construction, road base, and sometimes landscaping.

Geological facts

The spotted appearance is characteristic of porphyritic rocks. This specific specimen appears to be water-worn, smoothed by tumbling in a river or ocean environment.

Field identification & locations

Identifiable by the distinct lighter spots (phenocrysts) in a dark, fine-grained matrix. Commonly found in volcanic regions or in riverbeds/beaches where volcanic rock has been transported and smoothed.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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Aplite with Arfvedsonite inclusions

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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.

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Magnetite

Iron(II,III) oxide (Fe3O4)

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