igneous
Porphyritic Basalt (or Porphyry)
Porphyritic Mafic Volcanic Rock
Displays large, light-colored crystals (phenocrysts) embedded in a fine-grained, dark grey to black groundmass. Hardness: 6-7 on Mohs scale; Luster: Dull to sub-vitreous; Texture: Porphyritic.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Displays large, light-colored crystals (phenocrysts) embedded in a fine-grained, dark grey to black groundmass. Hardness: 6-7 on Mohs scale; Luster: Dull to sub-vitreous; Texture: Porphyritic.
Formation & geological history
Formed from two-stage cooling of magma. Initial slow cooling deep underground allowed large feldspar crystals to grow, followed by rapid eruption and cooling of the remaining melt as lava on the surface.
Uses & applications
Commonly used as crushed stone for road construction, railroad ballast, and decorative garden landscaping due to its durability and unique appearance.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple. These rocks are often referred to as 'Peanut Brittle Rock' by amateur collectors because of the scattered white crystals resembling nuts.
Field identification & locations
Identify by looking for distinct, isolated rectangular or oval crystals surrounded by a uniform, dark, fine-grained matrix. Often found in volcanic regions or as glacial erratics.
