igneous
Porphyry
Porphyritic Andesite or Rhyolite
AI-generated identification · may be incorrectHardness: 6-7 (Mohs scale), Color: Gray to green groundmass with white phenocrysts, Luster: Dull/Matte, Crystal Structure: Porphyritic (large crystals in a fine-grained matrix), Specific Gravity: 2.5-2.8
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Gray to green groundmass with white phenocrysts, Luster: Dull/Matte, Crystal Structure: Porphyritic (large crystals in a fine-grained matrix), Specific Gravity: 2.5-2.8
Formation & geological history
Formed when magma cools in two stages: first, large crystals (phenocrysts) grow slowly underground; second, the remaining magma cools rapidly near or at the surface. These specimens are common in volcanic arcs and mountain belts.
Uses & applications
Historically used for ornamental sculpture and monumental architecture; currently used as crushed stone for road construction and decorative landscaping.
Geological facts
The term 'Porphyry' comes from the Greek word for purple, as the most famous ancient variety found in Egypt was deep purple with white spots and reserved for Roman Imperial use.
Field identification & locations
Identify by looking for distinct, isolated large crystals (often feldspar) 'floating' in a much finer-grained background. Commonly found in riverbeds and glacial deposits in volcanic regions.
