igneous
Porphyry
Porphyritic Basalt or Andesite
Hardness: 6-7 (Mohs scale); Color: Dark grey to black groundmass with light-colored (white to pinkish) crystals; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic groundmass with phenocrysts; Specific Gravity: 2.7-3.0.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to black groundmass with light-colored (white to pinkish) crystals; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic groundmass with phenocrysts; Specific Gravity: 2.7-3.0.
Formation & geological history
Formed through a two-stage cooling process of magma: slow cooling deep underground creates large phenocrysts, followed by rapid cooling during a volcanic eruption creating the fine-grained matrix. Often associated with Proterozoic or Mesozoic volcanic activity.
Uses & applications
Primarily used as an ornamental stone, for architectural cladding, high-end paving, and historical monuments. Some specimens are used in jewelry as cabochons.
Geological facts
The word 'Porphyry' comes from the Ancient Greek word for purple. Historically, Imperial Porphyry was highly prized by Roman emperors and was only found in one remote Egyptian mine.
Field identification & locations
Identify by 'polka dot' appearance of large, distinct rectangular or angular crystals (phenocrysts) embedded in a microcrystalline background. Commonly found in glaciated regions (as eratics) or volcanic fields like the Cascades or Lake Superior region.
