I certainly am no chemist, but if what you are saying is true then I would think that the outside of the stones would have more die on/in them then the center of the stones. When you mop up liquids with a sponge it doesn't necessarily soak the whole sponge. If the dye is coming into contact with the stones after they they have been formed, I wouldn't think that you would have an even distribution of the dye. Maybe someone should try a test where they submerge some of their stones in a mixture containing blue dye for a period of time and then cut them open and see what they look like.