Rhyolite Pebble

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Grayish-pink to reddish-tan; Luster: Dull/Earthy; Crystal Structure: Aphanitic (fine-grained matrix); Cleavage: None/Irregular fracture; Specific Gravity: 2.4-2.6

Formation & Geological History

Formed through the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. This specific specimen has been smoothed by water erosion, likely in a river or coastal environment.

Uses & Applications

Commonly used as crushed stone for construction, road base, and landscape decoration. Polished pebbles are sometimes used in hobbyist jewelry or aquarium decor.

Geological Facts

Rhyolite has the same chemical composition as granite but cooled much faster, preventing large crystals from forming. Its high silica content makes the lava very viscous and prone to explosive eruptions.

Field Identification & Locations

Identify in the field by its fine-grained texture, light color (felsic), and its inability to be scratched by a steel knife. It is common in volcanic regions like the American West or Iceland.

Identified on: 4/25/2026

Mode: Standard