igneous
Porphyry
Porphyritic Rhyolite/Andesite
Hardness: 6-7 Mohs; Color: Dark grey to reddish matrix with light pink/white crystals; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with large crystals); Specific Gravity: 2.5-2.8
- Hardness
- 6-7 Mohs
- Color
- Dark grey to reddish matrix with light pink/white crystals
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to reddish matrix with light pink/white crystals; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with large crystals); Specific Gravity: 2.5-2.8
Formation & geological history
Formed through a two-stage cooling process of magma: slow cooling at depth creates large crystals (phenocrysts), followed by rapid cooling during eruption to form the fine-grained matrix.
Uses & applications
Used as decorative stone, in construction as crushed aggregate, and historically for statues and columns in ancient Rome.
Geological facts
The word 'porphyry' comes from the Ancient Greek word for purple. Imperial Porphyry was highly prized by Roman emperors for its deep purple color and extreme durability.
Field identification & locations
Look for distinct, visible crystals (usually feldspar) embedded in a much finer-grained, solid-colored background. Often found in volcanic regions or glacial till.
