Rhyolite is a fine-grained, volcanic rock, but if it (or dacite) cool so quickly that no crystals form, then obsidian is the result.
(Rhyolite; is; a fine-grained, volcanic rock) (it (or dacite; cool so quickly; that no crystals form)
Rhyolite it (or dacite
is cool so quickly
a fine-grained, volcanic rock
that no crystals form