igneous
Porphyry
Porphyritic Basalt or Andesite
Hardness: 6-7 (Mohs scale); Color: Dark brown or black matrix with light-colored tan/grey phenocrysts; Luster: Dull to waxy; Texture: Porphyritic (large crystals in fine-grained groundmass); Specific gravity: 2.5-3.0
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to waxy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark brown or black matrix with light-colored tan/grey phenocrysts; Luster: Dull to waxy; Texture: Porphyritic (large crystals in fine-grained groundmass); Specific gravity: 2.5-3.0
Formation & geological history
Formed via a two-stage cooling process of magma: initial slow cooling deep underground allows large crystals (phenocrysts) to grow, followed by rapid eruption and cooling of the remaining melt into a fine-grained matrix.
Uses & applications
Traditionally used in ancient road paving, ornamental architecture, sculpture, and modern construction aggregate; also popular as tumbled pocket stones or jewelry.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple, referring to a specific purple-red variety used by Roman emperors for statues and sarcophagi to symbolize royalty.
Field identification & locations
Identified by the distinct 'spotted' appearance of large, visible crystals embedded in a much finer-grained background. Commonly found in volcanic regions or as glaciated river stones.
