igneous
Rhyolite Porphyry
Rhyolite (High-silica volcanic rock)
Hardness: 6-7 (Mohs); Color: Pinkish-red to purple with white/grey phenocrysts; Luster: Dull to earthy; Texture: Porphyritic (fine-grained matrix with larger crystals); Specific gravity: 2.4-2.6
- Hardness
- 6-7 (Mohs)
- Color
- Pinkish-red to purple with white/grey phenocrysts
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Pinkish-red to purple with white/grey phenocrysts; Luster: Dull to earthy; Texture: Porphyritic (fine-grained matrix with larger crystals); Specific gravity: 2.4-2.6
Formation & geological history
Formed from rapid cooling of silica-rich lava at or near the Earth's surface. The larger crystals (phenocrysts) grew slowly at depth before the final eruption. Often found in continental volcanic arcs.
Uses & applications
Used in construction as crushed stone, road ballast, and occasionally as decorative landscaping stone or for cabochons if the pattern is particularly striking.
Geological facts
Rhyolite is the volcanic equivalent of granite. Because of its high silica content, rhyolite lava is very viscous (thick), which often leads to explosive volcanic eruptions rather than flowing lavas.
Field identification & locations
Identify by looking for 'spots' (phenocrysts) of quartz or feldspar set in a very fine-grained, colorful stony base. Common in the Western United States, particularly in areas with a history of explosive volcanism.
