The third coin pictured is my favorite.

However I am always tempted to think that the beautiful rainbow toning may be artificial by careful heat treatment.
For me there is no loss or increase in value for an
ASE bullion coin that
may? have been interfered with in this way, but others may not be so tolerant.
I I was a third party grader there would be no comment, because there no way to know for sure if the toning was natural or artificial.
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I think that I might try to experiment with a few newly issued copper nickel Australian coins. (20 and 50 Cent coins).
My plan is to experiment with a range heat treatment temperatures and exposure times. After each coin has cooled I will permanently fix the toning by sealing them in can spray acrylic, and then to protect the seal coat from chipping, to further protecting the coin in a snap lock acrylic capsule.
That way, the rainbow toning will be permanently fixed and protected.
High class 'shed job', with a touch of panache!

Beautiful rainbow toning can be unstable, and most probably deteriorate over time, hence the need for careful protection.
I have seen some nice examples of multi colored enameled British silver Half Crowns of the 19th Century.