igneous

Basalt with Plagioclase Phenocrysts (Porphyritic Basalt)

Mafic volcanic rock (formula primarily (Mg, Fe)2SiO4 and CaAl2Si2O8)

Hardness: 6 (Mohs scale), Color: Dark gray to black matrix with light cream or greenish-tan crystals, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic (fine-grained) matrix with visible Porpyhritic crystals (phenocrysts), Cleavage: None/Irregular

Identified March 27, 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 larger light-colored crystals (phenocrysts) formed slowly underground before the final eruption. Common throughout various geological eras from the Archean to the Holocene.

Uses & applications

Commonly used as crushed stone for aggregate in construction, road base, and railroad ballast. Exceptionally smooth or uniquely patterned specimens are occasionally tumbled for decorative 'river stones' or used in aquarium landscaping.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large phenocrysts in basalt indicate a 'two-stage' cooling history: a slow start deep underground followed by rapid cooling upon eruption.

Field identification & locations

Can be identified in the field by its high density (heavy for its size), dark color, and the presence of light-colored feldspar crystals embedded in a dark, fine-grained mass. Often found on beaches, near volcanic ranges, or in glacial till.

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