This was in a mint set and the cello was cracked. I’m assuming a liquid seeped in and caused the toning.
The mint set was left in a cars glove compartment in hot weather...no liquid needed the plastic baked the coin and the chemicals in the plastic did the rest.
Ive seen it go both ways...yes sometimes and no the other...broken plastic... is it brittle? I have seen a dealer leave a set in his car and toned just like yours...from excessive heat. Like baking a coin in an oven.
Not nessary as the coins color and blotchy tone it most probably will not grade... Plus you have to find a buyer who likes that sort of toning. ....not everyone does.
The reason I ask...is the mint over the years has had issues with their materials used to package a coin has turned the coins...sometimes all sometimes just a few. The 1994 & 1997 matte finished nickels turned gold on their reverses...the same for the west ward Jefferson set toned the same...the two silver dollars in these sets got a nice rainbow tone that actually looked like a rainbow over the obv top of the coin. To answer why the Kennedy toned and the others didnt...well different composition of medals, purchased from different vendors..... but im sure there are other members here who have experienced Mint packages toning their coins. Ill bet the only 1994 & 97 matte finished nickels the 2 lowest mintages in the series that arent toned were pulled Out sent for grading within 6 months of issue.
White. There’s specks of something on the surface. I believe it’s rust. Some of it was removed with acetone and a q-tip and it left those white looking specks. There is still some remaining.
Here are the sets I refered to notice the nickels dark and gold toned and the silver dollar has a rainbow tone around the rim.
Look at toms face see how there's lighter areas and other areas toned darker? All caused by mint packaging.....