While granite has crystals that are generally easy to see, in rhyolite the crystals are often too small to see. This is due to the more rapid cooling of the rhyolite lava compared to granite's slower cooling magma. In general, the slower a magma cools the larger the crystal size. Although crystals in rhyolite are usually hard to see, they are there, but as microscopic crystals often surrounded by a glassy matrix. If the lava fails to form crystals and is essentially all glass, then it is more correctly called obsidian.