Saundra, I don't believe that you can be sure that your mortar will stick to the old lava rock. Since your lava rock is probably just a facing also, you might be much better off to remove that and get down to the material that your fireplace is actually made from. I think that you should call in a pro mason, if nothing else but to get a correct basis for what you have and what should be done. With some guidance, you still may be able to do some or most of this yourself. At the same time, you may easily be saving yourself a lot of big and expensive problems in the future.