Rhyolite

Rhyolite is a natural rock, not a single mineral: an extrusive (volcanic) igneous rock with a very high silica content, the fine-grained equivalent of granite. It is made up mainly of quartz and feldspar (alkali feldspar such as sanidine, and plagioclase), with minor hornblende and biotite, in grains usually too small to see without a hand lens. Rhyolites with flow banding or spherulites are the ones most used as gem rocks.