Rhyolite is an extrusive igneous stone formed in volcanic eruptions where granitic magma reaches the surface.  It tends to be very viscous and, during eruptions, is more likely to be ejected as pyroclastic rock rather than lava flows (which most often form basalt).  Rhyolite has a high silica content and, depending on how quickly it cools, it can be relatively fine-grained and suitable for making stone tools.  If rhyolite cools very rapidly, obsidian is formed.