igneous
Porphyry
Porphyritic Basalt or Andesite
Hardness: 6-7 Mohs; Color: Brownish-green matrix with pink to tan phenocrysts; Luster: Dull to sub-vitreous (shiny when wet); Structure: Porphyritic texture with large crystals in a fine-grained groundmass.
- Hardness
- 6-7 Mohs
- Color
- Brownish-green matrix with pink to tan phenocrysts
- Luster
- Dull to sub-vitreous (shiny when wet)
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Physical properties
Hardness: 6-7 Mohs; Color: Brownish-green matrix with pink to tan phenocrysts; Luster: Dull to sub-vitreous (shiny when wet); Structure: Porphyritic texture with large crystals in a fine-grained groundmass.
Formation & geological history
Formed when magma cools in two stages: first, slowly deep underground creating large crystals (phenocrysts), then rapidly near or at the surface during a volcanic eruption.
Uses & applications
Used primarily in construction as crushed stone or road gravel, and historically as a prestigious decorative stone for sculptures and columns in Ancient Rome.
Geological facts
The word 'porphyry' comes from the Greek word for purple. Ancient Imperial Porphyry from Egypt was so prized that it was reserved exclusively for Emperors.
Field identification & locations
Identify by looking for distinct, isolated crystals embedded in a much finer-grained background. This specimen is a water-worn river pebble, likely found in volcanic regions or glacial till.
