Rainbow toning on a Gem coin is the most expensive thing by weight in the world. That said, even TPGers never always get it right when it comes to toning. On circ silver coins, polished surfaces can disappear in a minute, never to be seen again. Impregnated toning paper can cover the sins of cleaning and over-dipping. Study, study again, and study some more; know the differences between applied and naturally-spreading toning. The Best Thing that you can hope for is to look at a toned coin and know that something... isn't.... right if it isn't