igneous
Porphyry
Porphyritic Andesite/Rhyolite
Hardness (Mohs scale): 6-7, Color: reddish-brown or purple groundmass with white/grey crystals, Luster: dull to vitreous, Structure: porphyritic (large crystals in a fine matrix)
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Physical properties
Hardness (Mohs scale): 6-7, Color: reddish-brown or purple groundmass with white/grey crystals, Luster: dull to vitreous, Structure: porphyritic (large crystals in a fine matrix)
Formation & geological history
Formed from cooling magma in two stages: slow cooling at depth for the large phenocrysts, followed by rapid cooling during eruption for the fine matrix.
Uses & applications
Historically used for royal sculpture (Imperial Porphyry), construction, road gravel, and as a decorative stone in architecture and jewelry.
Geological facts
The term porphyry comes from the Greek word for purple. Ancient Romans used purple porphyry to symbolize the absolute power of the Emperor.
Field identification & locations
Identify by the 'chocolate chip cookie' appearance of large visible crystals (phenocrysts) suspended in a uniform, fine-grained mass. Often found in volcanic terrains.
