Rhyolite

Rhyolite forms when granitic magma erupts and cools quickly, usually in continental or continent-margin volcanic eruptions; it is rarely produced at oceanic eruptions, and most rhyolite occurs as lava flows. Gas trapped in the lava often leaves cavities (vugs) that can later host crystals, opal, agate or jasper. Much gem-quality rhyolite comes from Australia; a well-known US locality is near Truth or Consequences, Sierra County, New Mexico.

Rhyolite has the same general composition as granite but grains too small to distinguish by eye; geologists name hand specimens mainly by their quartz and salmon-colored K-feldspar phenocrysts. Trade names are applied loosely: sellers describe a pastel-green rock sold as "Rainforest Jasper" as rhyolite, banded rhyolite as "Wonderstone" (a term also misapplied to banded jasper and sandstone), and spherulitic rhyolite as "Birdseye". Liparite is an older European synonym. Obsidian and pumice have similar compositions but formed under different cooling conditions.

The name rhyolite, from the Greek rhyax (stream of lava), was coined in 1860 by Baron Ferdinand von Richthofen, alluding to the flow banding typical of the rock; the synonym liparite was introduced in 1861 by Justus Roth for rocks of the Lipari Islands. Where better materials were not available, people used rhyolite to make stone tools such as scrapers, blades and projectile points.