igneous
Porphyritic Rhyolite
Rhyolite
Hardness: 6-7 (Mohs scale); Color: Pinkish-red to reddish-brown; Luster: Pearly to dull with vitreous phenocrysts; Crystal structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None (conchoidal fracture)
- Hardness
- 6-7 (Mohs scale)
- Color
- Pinkish-red to reddish-brown
- Luster
- Pearly to dull with vitreous phenocrysts
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Pinkish-red to reddish-brown; Luster: Pearly to dull with vitreous phenocrysts; Crystal structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. The larger phenocrysts (visible crystals) formed cooling slowly underground before the final eruption. Common in volcanic regions across various geological ages.
Uses & applications
Used as decorative stone, crushed stone for construction and road base, and occasionally in jewelry as cabochons if the pattern is particularly striking.
Geological facts
Rhyolite is the volcanic equivalent of granite. The name comes from the Greek word 'rhyax', meaning a stream of lava. Because of its high silica content, rhyolitic lava is very viscous and often erupts explosively.
Field identification & locations
Identify by the reddish pink matrix containing visible quartz or feldspar crystals (phenocrysts). Commonly found in mountainous volcanic terrains such as the American Southwest or the Cascade Range.
