igneous
Rhyolite
Rhyolite (High-silica volcanic rock)
Hardness: 6-7 on Mohs scale; Color: Reddish-brown, pink, or gray; Luster: Dull/Earthy to Sub-glassy; Crystal structure: Aphanitic (fine-grained) or Porphyritic; Cleavage: None/Irregular fracture; Specific gravity: 2.4-2.6.
- Hardness
- 6-7 on Mohs scale
- Color
- Reddish-brown, pink, or gray
- Luster
- Dull/Earthy to Sub-glassy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Reddish-brown, pink, or gray; Luster: Dull/Earthy to Sub-glassy; Crystal structure: Aphanitic (fine-grained) or Porphyritic; Cleavage: None/Irregular fracture; Specific gravity: 2.4-2.6.
Formation & geological history
Formed through the rapid cooling of high-silica (felsic) magma at or near the Earth's surface. This specimen appears to be from a volcanic flow or tuff deposit and can range in age from Recent to hundreds of millions of years old.
Uses & applications
Used in construction as crushed stone or road fill. Occasionally used as decorative stone in landscaping or for making stone tools (historically). Higher-grade or patterned versions (like Bird's Eye Rhyolite) are used in jewelry.
Geological facts
Rhyolite is the volcanic equivalent of granite. Because of its high silica content, rhyolitic lava is extremely viscous and can lead to explosive volcanic eruptions rather than flowing like basalt.
Field identification & locations
Identify by its light-to-reddish color, fine-grained texture, and high hardness (it will scratch glass). It is commonly found in continental volcanic regions such as the American Southwest and the Rocky Mountains.
