igneous
Porphyry (Rhyolite or Andesite Porphyry)
Porphyritic Igneous Rock
Hardness: 6-7 on the Mohs scale; Color: reddish-purple to dark brownish-red groundmass with light cream to white plagioclase feldspar phenocrysts; Luster: dull to earthy matrix with sub-vitreous crystals; Cleavage: poor in mass; Specific Gravity: 2.5-2.8.
- Hardness
- 6-7 on the Mohs scale
- Luster
- dull to earthy matrix with sub-vitreous crystals
Identified More igneous →
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Physical properties
Hardness: 6-7 on the Mohs scale; Color: reddish-purple to dark brownish-red groundmass with light cream to white plagioclase feldspar phenocrysts; Luster: dull to earthy matrix with sub-vitreous crystals; Cleavage: poor in mass; Specific Gravity: 2.5-2.8.
Formation & geological history
Formed through a two-stage cooling process of magma. Large feldspar crystals (phenocrysts) formed slowly underground at depth, after which the magma erupted or intruded higher up, cooling quickly to create the fine-grained reddish matrix surrounding the crystals.
Uses & applications
Used historically and modernly as decorative dimension stone, crushed stone for road construction, landscape rock, and as lapidary material for polished stones or carvings.
Geological facts
Porphyry was highly valued by ancient Romans, especially Imperial Red Porphyry from Egypt, which was reserved exclusively for imperial statues, sarcophagi, and architectural columns due to its regal purple-red color.
Field identification & locations
Identified by its distinctive porphyritic texture—clearly visible, lighter-colored rectangular mineral crystals embedded within a fine-grained reddish-purple background matrix. Commonly found in ancient volcanic and intrusive igneous terranes worldwide, such as Lake Superior regions, Egypt, Europe, and western North America.
