Well the source explains why there are smells coming from it....it was likely previously live rock so there could be all kind of critters inside the rock that have long since died.
You might have a long road of soaking it and changing the water until you don't have any more smells coming off of it. The peroxide dip couldn't hurt and I would at least keep soaking it for a few more weeks with frequent water changes. Eventually all the trapped particles etc will completely decompose and then it will be ready to use. Then you will have a cool rock all full of the benficial bacteria you need.
Best wishes to you, I am sure it will be a really cool looking piece when you are done with it.
