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
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Physical properties
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
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.
