Probably. Scrub it well with a stiff brush first. Depending on what kind of rock it is, it may grow interesting algae. My snails love the rocks in our tank!
I would give the rock a good boil first. And you can do a vinegar test on any foreign object to determine if it is inert. Drop some plain vinegar on the cleaned dry rock - if it doesn't fizz, it's probably safe for your aquarium.
