igneous
Porphyry (Imperial Porphyry / Andesite Porphyry)
Porphyritic Andesite / Porphyritic Dacite
AI-generated identification · may be incorrectHardness 6-7 on Mohs scale; deep reddish-purple to burgundy-brown fine-grained groundmass with conspicuous white, cream, and greenish phenocrysts of plagioclase feldspar; vitreous to sub-vitreous luster when polished; specific gravity roughly 2.6-2.8; massive texture.
Identified More igneous →
Explore Porphyry (Imperial Porphyry / Andesite Porphyry) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness 6-7 on Mohs scale; deep reddish-purple to burgundy-brown fine-grained groundmass with conspicuous white, cream, and greenish phenocrysts of plagioclase feldspar; vitreous to sub-vitreous luster when polished; specific gravity roughly 2.6-2.8; massive texture.
Formation & geological history
Formed through a two-stage cooling of volcanic/subvolcanic magma. Larger phenocrysts crystallized slowly at depth, followed by rapid cooling of the remaining melt near or at the surface, typically in volcanic arc systems during Precambrian to Tertiary periods.
Uses & applications
Carved into decorative objects such as polished eggs, spheres, and cabochons. Historically, purple porphyry was a prestigious royal and imperial stone used for Roman monuments, imperial sarcophagi, and architectural columns.
Geological facts
Red or purple porphyry ('porphyry' derived from the Greek word for purple) was so revered in Ancient Rome that it became a symbol of imperial majesty, exclusively reserved for emperors and sacred statuary.
Field identification & locations
Identified in the field by its distinct porphyritic texture (obvious visible crystal grains suspended in an aphanitic, fine-grained matrix), high hardness, and reddish-purple matrix. Found in Egypt (Gebel Dokhan), Greece, Sweden, and various volcanic terranes worldwide.
