igneous
Porphyry pebble
Porphyritic Andesite / Porphyry
Hardness: 6-7 on Mohs scale. Color: Blue-grey matrix with cream, white, or light tan phenocrysts. Luster: Dull to smooth (water-worn). Texture: Porphyritic (visible crystals embedded in a fine-grained groundmass).
- Hardness
- 6-7 on Mohs scale
- Color
- Blue-grey matrix with cream, white, or light tan phenocrysts
- Luster
- Dull to smooth (water-worn)
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Blue-grey matrix with cream, white, or light tan phenocrysts. Luster: Dull to smooth (water-worn). Texture: Porphyritic (visible crystals embedded in a fine-grained groundmass).
Formation & geological history
Formed when magma cools slowly deep underground, allowing larger crystals (phenocrysts, usually plagioclase feldspar) to grow, followed by rapid cooling as it rises closer to the surface, forming a fine-grained matrix. Smooth pebble shape created by river or beach erosion.
Uses & applications
Landscaping, decorative pebbles, lapidary projects, and geological collecting.
Geological facts
Porphyritic textures record a two-stage cooling history of igneous rocks. The word 'porphyry' comes from the Ancient Greek word for purple, as the original prized imperial porphyry was deep red-purple.
Field identification & locations
Identified by the distinct contrast between large, light-colored feldspar crystals ('flecks') and a dark, fine-grained background matrix. Commonly found along gravel riverbeds, glacial tills, and beaches.
