igneous

Porphyritic Basalt (or Porphyry)

Porphyritic Basalt (Mafic volcanic rock with phenocrysts)

Hardness: 6 (Mohs scale); Color: Dark grey to reddish-brown groundmass with white or light-colored crystals; Luster: Dull/earthy (matrix) to glassy (phenocrysts); Structure: Porphyritic (large crystals in a fine-grained matrix); Cleavage: None (conchoidal fracture common).

Hardness: 6 (Mohs scale)
Luster: Dull/earthy (matrix) to glassy (phenocrysts)

Identified March 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed through a two-stage cooling process of lava or shallow magma. Larger crystals (phenocrysts) form slowly deep underground, while the fine-grained matrix cools rapidly after an eruption or intrusion near the surface. Commonly found in volcanic arcs and oceanic crust areas.

Uses & applications

Used primarily in industrial construction as high-quality road aggregate, railway ballast, and occasionally as decorative facing stone or paving sets.

Geological facts

The term 'porphyry' comes from the Ancient Greek word for purple, referring to the prized reddish-purple variety used by Roman emperors for statues and columns. It is the classic example of bimodal cooling rates in geology.

Field identification & locations

Identify by looking for 'spots' (large crystals) embedded in a solid, fine-grained rock. Found in basaltic lava flows and volcanic complexes globally (e.g., Cascades, Hawaii, Andes). Collectors value samples where crystalline faces are well-defined.

Other igneous specimens

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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)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral