You can get one of those little propane torches. Set the piece of rock on something that is not flammable, preferably outside depending on what nasty smells you like in your house. Place it so you can get a good shot at what you want to remove and then flame on.
You should be able to tell when the offending item is toasted for good. Or you could just keep the heat on until it quits smoking, then you know all of the moisture has been cooked out of it. You may want to try to brush it off the rock a little before placing it back in the tank.
