Photo: Carl Bento/Australian Museum
Surely you've seen fossils in museums, but what about this: opalized dinosaur tooth. Opalized fossils occur when silica settled into cracks in the dinosaur bone and then hardened into opal. This one above is a particularly fantastic specimen: an opalized theropod dinosaur tooth from the Australian Museum.
http://www.ucmp.berkeley.edu/diapsids/saurischia/theropoda.html