igneous
Porphyry
Porphyritic Rhyolite/Andesite
Hardness: 6-7 (Mohs); Color: Light grey or blue-grey matrix with darker phenocrysts; Luster: Dull to earthy (matrix), vitreous (crystals); Structure: Porphyritic (large crystals in fine-grained groundmass)
- Hardness
- 6-7 (Mohs)
- Color
- Light grey or blue-grey matrix with darker phenocrysts
- Luster
- Dull to earthy (matrix), vitreous (crystals)
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Physical properties
Hardness: 6-7 (Mohs); Color: Light grey or blue-grey matrix with darker phenocrysts; Luster: Dull to earthy (matrix), vitreous (crystals); Structure: Porphyritic (large crystals in fine-grained groundmass)
Formation & geological history
Formed during two stages of cooling: first deep underground creating large crystals (phenocrysts), then rapid cooling after eruption or shallow intrusion creating the fine-grained matrix.
Uses & applications
Commonly used as crushed stone for road construction, decorative garden stone, and historical building material (especially the famous Imperial Porphyry).
Geological facts
The term porphyry comes from the Greek word for purple. The Romans highly prized a deep purple variety found in Egypt, using it for statues and sarcophagi of emperors.
Field identification & locations
Identify by looking for distinct, larger visible crystals embedded in a uniform, much finer-grained rock body. It is often found in volcanic arc environments.
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