Rhyolite with Lithophysae and Iron Oxide

Hardness: 6 on the Mohs scale. Color: Pinkish-grey groundmass with dark brown to reddish-brown iron oxide (hematite/goethite) and white chalcedony or quartz infillings. Luster: Dull to vitreous in crystalline cavities. Crystal structure: Microcrystalline to cryptocrystalline groundmass with distinct vugs and mineral fillings. Cleavage: None (conchoidal fracture). Specific gravity: 2.35 - 2.60.

Formed from rapidly cooling silica-rich lava extruded on or near the Earth's surface. Volcanic gases escaping through cooling pockets create cavities (lithophysae or vugs) which are later filled with hydrothermal fluids carrying quartz, chalcedony, and iron oxides.

Used as decorative landscape rock, occasionally cut and polished into cabochons for jewelry, and studied by geologists as indicators of ancient volcanic activity.

Rhyolite is the extrusive equivalent of granite, having the same mineral composition (primarily quartz, sanidine, and plagioclase) but a very fine-grained texture due to rapid cooling on the surface.

Can be identified in the field by its fine-grained felsic matrix, often showing flow banding or gas cavities containing secondary mineral deposits like quartz or iron oxides. Found in volcanic regions worldwide.