igneous
Porphyry
Porphyritic Basaltic Andesite
Hardness: 6-7 (Mohs scale); Color: Dark gray to greenish-black matrix with white or light cream phenocrysts; Luster: Dull to vitreous; Crystal Structure: Porphyritic (large crystals in fine-grained groundmass); Specific Gravity: 2.5-3.0
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to vitreous
Identified More igneous →
Explore Porphyry in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark gray to greenish-black matrix with white or light cream phenocrysts; Luster: Dull to vitreous; Crystal Structure: Porphyritic (large crystals in fine-grained groundmass); Specific Gravity: 2.5-3.0
Formation & geological history
Formed from molten rock cooling in two stages: initially cooling slowly deep underground to form large crystals, then erupting or moving closer to the surface to cool rapidly, trapping crystals in a fine-grained matrix.
Uses & applications
Used primarily as decorative stone, crushed stone for construction, road basework, and historically for statues and monuments due to its durability.
Geological facts
The word 'Porphyry' comes from the Ancient Greek word for purple. Imperial Porphyry was highly prized in the Roman Empire and was reserved for the use of emperors for their sarcophagi and palace decorations.
Field identification & locations
Identify in the field by looking for distinct, isolated, light-colored crystals (phenocrysts) set against a darker, uniform background rock. Commonly found in volcanic regions or as rounded river cobbles.
