igneous
Porphyry
Porphyritic Igneous Rock
Hardness: 6-7 (Mohs scale); Color: Dark grey or green matrix with conspicuous pink or tan phenocrysts; Luster: Dull to vitreous; Crystal structure: Aphanitic (fine-grained) groundmass with large, visible crystals; Specific gravity: 2.5-3.0.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey or green matrix with conspicuous pink or tan phenocrysts; Luster: Dull to vitreous; Crystal structure: Aphanitic (fine-grained) groundmass with large, visible crystals; Specific gravity: 2.5-3.0.
Formation & geological history
Formed through a two-stage cooling process of magma: initial slow cooling deep underground creates large crystals (phenocrysts), followed by rapid cooling during eruption or intrusion near the surface to form the fine-grained matrix.
Uses & applications
Used primarily in construction as crushed stone or dimension stone. Historically used in ornate sculpture and high-end architecture (especially the deep red 'Imperial Porphyry' in Ancient Rome). Highly valued by collectors for its aesthetic 'polka-dot' appearance.
Geological facts
The word 'porphyry' comes from the Greek word for purple. In the Roman Empire, certain types were symbolisms of extreme wealth and high status, reserved specifically for the Emperor's use.
Field identification & locations
Identify in the field by looking for distinct, isolated crystals (often feldspar) that contrast sharply with a dense, fine-grained background rock. Commonly found in volcanic arcs or areas with ancient tectonic activity. Water-worn specimens are frequent in glacial till or riverbeds.
