igneous

Basalt with Phenocrysts (Porphyritic Basalt)

Porphyritic Basalt (primarily Plagioclase and Pyroxene)

Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with light-colored inclusions; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) groundmass with visible crystals (phenocrysts); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.

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

Identified March 14, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The distinct 'spots' or phenocrysts formed slowly underground before a final eruption caused the remaining lava to cool quickly. Common in volcanic regions and ocean floors.

Uses & applications

Used primarily in construction for road base, concrete aggregate, and railroad ballast. Larger porphyritic specimens are often used for decorative landscaping or architectural facings.

Geological facts

Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. Porphyritic varieties like this one provide a visual record of a 'two-stage' cooling process in a volcano's history.

Field identification & locations

Identify in the field by its dark, dense matrix and small, embedded light-colored crystals (usually feldspar). This specimen appears water-worn, likely found in a riverbed or on a beach. Look for it in areas with high past or present volcanic activity like the Pacific Northwest or Iceland.

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