igneous
Porphyry
Porphyritic Igneous Rock (likely Porphyritic Andesite or Basalt)
Hardness: 6-7 Mohs; Color: Dark grey to black groundmass with lighter grey to greenish phenocrysts (crystals); Luster: Sub-vitreous to dull; Crystal structure: Aphanitic (fine-grained) base with phaneritic (visible) crystals; Specific gravity: 2.5-3.0.
- Hardness
- 6-7 Mohs
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to black groundmass with lighter grey to greenish phenocrysts (crystals); Luster: Sub-vitreous to dull; Crystal structure: Aphanitic (fine-grained) base with phaneritic (visible) crystals; Specific gravity: 2.5-3.0.
Formation & geological history
Formed in a two-stage cooling process of magma: large crystals (phenocrysts) form slowly deep underground, and the remaining liquid cools rapidly upon eruption or rising, creating the fine-grained matrix. Often associated with volcanic arcs and subduction zones.
Uses & applications
Used primarily as decorative stone, construction aggregate, and in monuments or sculptures due to its interesting texture and durability. Historically prized by Romans for carvings.
Geological facts
The word 'Porphyry' comes from the Greek word for 'purple', as the most famous ancient specimens were deep red or purple. It was once reserved exclusively for imperial Roman use.
Field identification & locations
Identify in the field by looking for 'spots' (large crystals) embedded in a much finer-grained, solid-colored rock. These specimens are often found in glacially deposited gravel or near volcanic regions. For collectors, look for high contrast between the background and the crystals.
