Of course Rainbow toned coins carry a premium but only if they are uncirculated, naturally toned, and certified by PCGS, NGC, or ANACS. It depends on the colors and the look of the coin. People will bid high on one like it's an art piece. If you go to the PCGS coin forum, a significant amount of topics are related to toned coins. If you look at Feebay, uncirculated ones certified in slabs sell for moon money. I have dealt with this issue for years or almost a decade. I have tried over and over to tone coins faster for a natural toned look. It's extremely hard to do. I've done all the stuff, heating them, putting them with boiled eggs, and bought books on toning metals. To this day, I realize it is something you cannot do over night for the natural look. It is easy for someone with a trained eye to spot an artificially toned coin. Getting them certified is the hardest part. You may pay money for grading fees and it not get graded because they deemed it to have artificial toning. PCGS also has a new thing called the sniffer that they can do a chemical analysis and see if any chemicals were applied to the coin.
I have to admit, I sold a couple modern kennedy halves that I heated for a blue look to people through Feebay years ago. It was not a significant premium but I made a couple bucks on it. I did not feel good about selling them that I stopped doing it since it was artificial toning. I just said the coins were toned blue and people wanted them. It suprised me. People on the PCGS forum are always pointing out the new artificial toner coin doctor on Feebay. I didn't want to be one of them.
Now I just have a couple silver uncirculated coins sitting in old holders like Wayte Raymond and they're toning away the natural way. However, they've been sitting there for years and the color progression has been very slow. Another issue is that not all coins tone or if they do, the toning might be ugly, spotty, and unattractive. That's why nicely natural toned ones have such a premium. You really appreciate the beauty of natural rainbow toning if you've had them for so long and they've toned naturally.
