igneous

Porphyry

Porphyritic Rhyolite or Andesite

Hardness: 6-7 (Mohs scale); Color: Brownish-green matrix with white phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic matrix with larger visible crystals (phenocrysts); Cleavage: None for the rock; Specific Gravity: 2.5-2.8

Hardness: 6-7 (Mohs scale)
Color: Brownish-green matrix with white phenocrysts
Luster: Dull to sub-vitreous

Identified April 5, 2026More igneous →

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Physical properties

Formation & geological history

Formed through a two-stage cooling process of magma; larger crystals formed slowly underground before a final rapid cooling period on or near the surface trapped them in a fine-grained matrix. Common in volcanic arcs and tectonic margins.

Uses & applications

Used primarily as decorative aggregate, in construction for road base, and historically for statues and monuments (especially Imperial Porphyry from Egypt). Small river-worn specimens are popular with hobbyist collectors.

Geological facts

The word 'porphyry' comes from the Ancient Greek word for purple. Ancient Romans used a specific deep purple variety as a symbol of imperial rank and power, as it was exceedingly rare and difficult to carve.

Field identification & locations

Identified by the distinct contrast between large, well-formed crystals (phenocrysts) and the surrounding fine-grained background. In the field, look for a 'polka-dot' appearance in volcanic rock formations or riverbeds.

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