igneous
Banded Rhyolite (Wonderstone)
Rhyolite (High-silica volcanic rock)
Hardness: 6-7 Mohs; Color: Banded greens, tans, ochre, and grays; Luster: Dull to waxy; Texture: Fine-grained, aphanitic; Cleavage: None (conchoidal fracture).
- Hardness
- 6-7 Mohs
- Color
- Banded greens, tans, ochre, and grays
- Luster
- Dull to waxy
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Physical properties
Hardness: 6-7 Mohs; Color: Banded greens, tans, ochre, and grays; Luster: Dull to waxy; Texture: Fine-grained, aphanitic; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from rapid cooling of silica-rich lava. The banding or swirling patterns are caused by flows of viscous magma capturing different mineral impurities or by secondary staining from iron/manganese oxides during devitrification.
Uses & applications
Primarily used as a decorative stone for lapidary work, cabochons, carvings, and as a popular specimen for rock collectors.
Geological facts
The term 'Wonderstone' is often applied to beautifully banded rhyolites or tuffaceous stones found in the Western United States. The movement of the lava was so thick and slow that it preserved flow lines as visible stripes.
Field identification & locations
Identify by its matte finish (unless polished) and distinct, non-geometric flow-banding. Common in volcanic regions like Nevada, Utah, and Oregon. Collectors look for high contrast in the bands.
