Igneous

Basalt with Plagioclase Phenocrysts

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

Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with light white/tan inclusions; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) matrix with visible phenocrysts; Cleavage: None (conchoidal fracture).

Hardness: 6-7 on Mohs scale
Color: Dark grey to black matrix with light white/tan inclusions
Luster: Dull to sub-vitreous

Identified March 14, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The larger white crystals (phenocrysts) grew slowly at depth before the final eruption. Common throughout the Cenozoic era to the present.

Uses & applications

Used primarily as crushed stone for aggregate in construction, road base, and railroad ballast. Larger decorative specimens are used in landscaping and aquascaping.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Porphyritic varieties like this one provide a 'snapshot' of a volcano's plumbing system by showing minerals that crystallized at different temperatures.

Field identification & locations

Identify in the field by its high density (heavy for its size) and the contrast between the dark, fine-grained background and larger white feldspar crystals. Frequently found in volcanic fields, riverbeds draining volcanic highlands, and glacial till.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

mineral