igneous
Porphyry
Porphyritic Andesite/Basalt
Hardness: 6-7 (Mohs); Color: Dark grey to greenish-black groundmass with light-colored (white/cream) phenocrysts; Luster: Dull to sub-vitreous; Crystal Structure: Porphyritic (large crystals in a fine-grained matrix); Cleavage: Distinct in crystals, none in matrix.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to greenish-black groundmass with light-colored (white/cream) phenocrysts; Luster: Dull to sub-vitreous; Crystal Structure: Porphyritic (large crystals in a fine-grained matrix); Cleavage: Distinct in crystals, none in matrix.
Formation & geological history
Formed through a two-stage cooling process of magma; larger phenocrysts form slowly at depth, then the remaining liquid cools rapidly upon eruption or shallow intrusion. Common in volcanic arc environments.
Uses & applications
Used primarily as crushed stone for road construction, decorative garden stones, and historically as high-end ornamental building stone (Imperial Porphyry).
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple, as the most famous variety used by Roman emperors was deep purple with white spots.
Field identification & locations
Identified by the distinct 'spotted' appearance of large visible crystals (phenocrysts) embedded in a much finer-grained background. Found in riverbeds and glacial deposits in volcanic regions.
