Rhyolite

Rhyolite is the general term for a group of acidic igneous rocks, the effusive equivalent of granites. It occurs as lava flows, breccias, and in volcanic necks and dikes. In the porphyritic varieties the phenocrysts are frequently quartz or orthoclase feldspar imbedded in a highly felsitic or glassy ground mass. Rhyolites, including obsidian, frequently show flow, spherulitic, nodular, and lithophysal structures.

see also: List of minerals