igneous

Basalt with Plagioclase Phenocrysts (Porphyritic Basalt)

Porphyritic Basalt (primarily Plagioclase Feldspar, Pyroxene, and Olivine)

Hardness: 6 (Mohs scale); Color: Dark grey to charcoal matrix with white crystalline laths; Luster: Dull/Sub-metallic; Structure: Aphanitic matrix with visible phenocrysts; Cleavage: None (conchoidal fracture).

Hardness: 6 (Mohs scale)
Color: Dark grey to charcoal matrix with white crystalline laths
Luster: Dull/Sub-metallic

Identified March 23, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava on or near the Earth's surface. The white 'spots' (phenocrysts) formed deeper in the magma chamber before the eruption. Age varies by location but ubiquitous throughout Earth's history.

Uses & applications

Used primarily as crushed stone for construction, road base, concrete aggregate, and railroad ballast. Exceptionally durable for high-traffic paving stone.

Geological facts

Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. Porphyritic varieties are often called 'Chinese Writing Stone' when the feldspar crystals form patterns resembling calligraphy.

Field identification & locations

Identify by its heavy weight (high density), dark fine-grained groundmass, and distinct rectangular white crystals. Commonly found in volcanic regions, riverbeds near volcanic flows, and coastal areas.

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