Sorry for your loss. You have to be careful and know what the rock sample is.....some rock samples don't like water (will absorb it and break down). Soap will have a difficult time removing smoke. If the sample can handle it, TSP might be worth a try. Otherwise, I would visit a fireplace store and get their glass cleaner which is specially designed to remove wood smoke and creosote from hard surfaces including metal and glass. The problem of course with a house fire is there probably was all kinds of nasty stuff (plastic, rubber, etc) in the smoke film.