Rhyolite

Physical Properties

Hardness: 6-7 Mohs; Color: Pink, reddish-brown, or grey; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic or porphyritic (fine-grained matrix with larger phenocrysts); Cleavage: None; Specific Gravity: 2.4-2.6

Formation & Geological History

Formed through the rapid cooling of silica-rich (felsic) magma/lava at or near the Earth's surface. This specimen shows flow banding typical of high-viscosity lava flows.

Uses & Applications

Used as crushed stone for construction, road fill, and sometimes as an abrasive or in decorative landscaping. Certain colorful varieties are used for lapidary work.

Geological Facts

Rhyolite has the same chemical composition as granite but cooled much faster. Many famous geological features, like the columns at Devils Postpile or parts of Yellowstone National Park, are composed of rhyolitic rock.

Field Identification & Locations

Identify in the field by its fine-grained texture, light color (indicating high silica), and characteristic flow-banding or layering. Often found in continental volcanic arcs.

Identified on: 5/2/2026

Mode: Standard