igneous
Granite porphyry
Porphyritic Granite
Hardness: 6-7 (Mohs scale); Color: Black or dark grey groundmass with tan, pink, or white phenocrysts; Luster: Dull to vitreous; Structure: Porphyritic (large crystals in fine-grained matrix); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black or dark grey groundmass with tan, pink, or white phenocrysts; Luster: Dull to vitreous; Structure: Porphyritic (large crystals in fine-grained matrix); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the slow-to-rapid cooling of silica-rich magma under the Earth's surface. The large phenocrysts form during slow underground cooling, while the finer matrix forms during more rapid cooling as the magma moves upward.
Uses & applications
Used primarily as decorative stone, in construction as crushed stone or gravel, and occasionally for lapidary work or polishing for collectors.
Geological facts
The word 'porphyry' comes from the Ancient Greek word for 'purple', as the first described specimens were purple-red volcanic rocks from Egypt used in Roman imperial sculptures.
Field identification & locations
Look for distinct, large crystals (phenocrysts) embedded in a significantly finer-grained rocky background. Commonly found in mountainous regions or glacial deposits like riverbeds and beaches.
