igneous

Porphyritic Basalt

Porphyritic Fine-grained Mafic Volcanic Rock

Hardness: 6.0 (Mohs scale); Color: Dark grey to black matrix with white or creamy phenocrysts; Luster: Dull/earthy matrix with vitreous crystals; Crystal structure: Aphanitic matrix with larger visible crystals (phenocrysts); Cleavage: Poor; Specific gravity: 2.8-3.0

Hardness: 6
Color: Dark grey to black matrix with white or creamy phenocrysts
Luster: Dull/earthy matrix with vitreous crystals

Identified May 3, 2026More igneous →

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Physical properties

Formation & geological history

Formed through a two-stage cooling process of mafic lava. The larger crystals form slowly underground before the magma erupts, while the fine-grained matrix forms quickly upon reaching the Earth's surface.

Uses & applications

Commonly used as crushed stone for road construction, railroad ballast, and concrete aggregate. Select specimens are used as decorative garden stones or for architectural cladding.

Geological facts

The term 'porphyritic' comes from the Greek word for purple, originally referring to a specific purple Egyptian stone. This texture provides a visual timeline of a volcano's cooling history.

Field identification & locations

Identify in the field by looking for a dark, dense matrix containing distinct, larger crystals (often plagioclase feldspar). It is commonly found in oceanic crust and large volcanic provinces (flood basalts).

Other igneous specimens

See all

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