If the stone is too big and you're not using the right tool for cutting it, then just start from one side and cut as much of the stone away that way. This gives you a better chance at being able to get through all material without having to make multiple cuts or take more time than necessary when making this type of mistake. If you end up trimming lots of rough on a lapidary saw blade before starting your project but don't want to ruin either piece with too many facets then try cutting into slabs instead by going perpendicular across rock width (horizontal) so that both pieces are still usable even if they have a less finished product.