igneous

Porphyritic Basalt

Porphyritic Mafic Extrusive Rock

Hardness: 6 (Mohs); Color: Dark grey to black groundmass with light-colored phenocrysts; Luster: Dull (groundmass) to vitreous (crystals); Structure: Fine-grained aphanitic matrix with visible crystals (porphyritic); Specific gravity: 2.8 - 3.0

Hardness: 6 (Mohs)
Color: Dark grey to black groundmass with light-colored phenocrysts
Luster: Dull (groundmass) to vitreous (crystals)

Identified May 3, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava on the Earth's surface. The large crystals (phenocrysts) formed slowly during an initial underground phase before being erupted and encased in a fine-grained matrix.

Uses & applications

Commonly used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally durable specimens are used for pavers and tiles.

Geological facts

The word 'basalt' is derived from late Latin 'basaltes', a misspelling of 'basanites' meaning 'very hard stone'. This texture indicates a two-stage cooling process for the magma.

Field identification & locations

Identify by looking for a very dark, fine-grained background containing scattered, distinct, larger crystals (often plagioclase feldspar or olivine). Found in volcanic regions like Hawaii, Iceland, and the Deccan Traps.

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