igneous
Porphyry (Rhyolite / Andesite Porphyry Pebble)
Porphyritic Igneous Rock
Hardness: 6-7 on Mohs scale. Color: reddish-brown to dark gray matrix with visible lighter mineral phenocrysts (plagioclase or potassium feldspar). Luster: dull to earthy on weathered surfaces. Texture: porphyritic (aphanitic fine-grained matrix containing larger crystals).
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to earthy on weathered surfaces
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Physical properties
Hardness: 6-7 on Mohs scale. Color: reddish-brown to dark gray matrix with visible lighter mineral phenocrysts (plagioclase or potassium feldspar). Luster: dull to earthy on weathered surfaces. Texture: porphyritic (aphanitic fine-grained matrix containing larger crystals).
Formation & geological history
Formed when magma undergoes a two-stage cooling process: initial slow cooling deep underground allows larger feldspar or quartz crystals (phenocrysts) to form, followed by rapid eruption and cooling of the surrounding magma into a fine-grained matrix. Smooth, rounded pebble shape results from stream or beach erosion.
Uses & applications
Used historically as building stone, decorative landscaping gravel, and crushed stone aggregate. Well-rounded river pebbles are commonly collected as pocket stones.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple, as the most famous ancient decorative porphyry rocks (Imperial Porphyry from Egypt) were reddish-purple and reserved for Roman royalty.
Field identification & locations
Identified by the distinct 'chocolate chip' pattern where individual, visible feldspar or quartz crystals are set inside a much finer-grained, dark or reddish volcanic matrix. Very common along gravel riverbeds, lake shores (such as Lake Superior), and volcanic regions.
