Most soft stone, like that used for seal carving, can be bruised.If you get a bruise it will look dark but the opposite is true for stone. This means that if you hit a stone or drop it, the stone may not chip or break but will instead show a white spot where the stone has been compressed. I don’t know the technical details about the cause, but the cure is to soak the bruise in oil which may even out the spot. The only other option is to sand it out. So don’t drop your seal on a marble floor—you’ll bruise your seal stone and you’ll bruise your floor.