Rhyolite is a fine-grained extrusive igneous rock or volcanic rock. It is pale coloured, often light grey, tan or pinkish. Rhyolite is made up of quartz and feldspar crystals, and occasionally contains some mafic (dark coloured) minerals.
Usually the crystals are too small to see without magnification, but occasionally contains larger crystals, or small round pockets that were gas bubbles. Sometimes it can be banded.
How was it formed?
Rhyolite is a volcanic rock. It is fine-grained because it forms by the rapid cooling of magma, usually when it erupts onto the Earth's surface. When rhyolite erupts quietly it forms lava flows. If it erupts explosively it often forms pumice. Rhyolite forms from magma that contains lots of silica (quartz) and is the fine-grained equivalent of granite.