The difference is rhyolite has much finer crystals. ... Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite giving it a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende. Gabbros are dark-colored, coarse-grained intrusive igneous rocks.
This is from the website volcano.oregonstate.edu/book/export/html/1055
