2008-Silver Quarter From Silver Proof Set With Copper Coloring

Can't really tell you how or why it happened. Maybe where it was stored, excessive hot, cold, dampness, humidity, etc.. Environmentally issues?

Basically, welcome to the United States Mint quality control. I've seen much worse.

BTW: Those Proof sets are not air tight.

I didn't know it arrived that way. I would have sent it back. But you didn't know the difference, so I guess it's to late now.

toning usually either starts at the edge and works towards the center or starts at the center and works towards the edge.
can also be a contaminant from wherever the set was put together. but it didn't happen during the strike during the minting process. This would be one of those Quality control issues that should have been refused and returned for a refund or replacement when noticed.

These coins were struck at different times, definitely on different dies, maybe on different presses also. this isn't a mint striking problem. maybe someones greasy frenchfry fingers during packaging or something like that.

the 14 coin silver proof set sold in 2008 for $46.25, you can by clean ones now for close to that price still off ebay. Melt value for the silver in that set, probably around $30 or so.... I think would be an take-able offer at this point if you were looking to offload the set and buy a replacement,

you could just buy the 5 coin silver proof quarter lens instead though if that's all that has a problem for between $20-$25, and sell the silver quarters and break close to even, as I think you could get around $20 for 5 90% silver quarters. on the silver proof sets you take the hit on the clad dollars, cent and nickel, so If you can avoid breaking the set for the half and the dime, you can in theory buy a replacement quarter lens 2008S State Quarter silver 5 coin set to replace that lens with, then sell the ugly damaged one to recover maybe even all of your cost. or just lose a buck or two, maybe even gain a buck or two.... depending on the final prices.

which I guess brings me bacl to the initial post and that possibly this is a 5 coin quarter silver proof set. if that's the case, then I'd think if you watch ebay and pictures are accurate, you can get a replacement set for right around melt value anyways, then just buy a clean set and sell this one for melt nearby to be even.

I believe it's toning, and I believe from the pattern it was someone doing the packaging touching it to put it in the holder with a greasy/contaminated gloved finger.which I guess is better than a fingerprint etched into the center of the coin, but not much better really...

A member here, @BrandaBob, posted about a year ago an absolutely nasty Presidential dollar set from 2009. like someone had sneezed on it during packaging and sent it out the door. spots galore!
the thread was here, http://goccf.com/t/342460

Anyways, main point here is the silver quarter 5 coin set is selling on ebay for right around melt value of 5 90% quarters, so getting a replacement shouldn't cost you all that much if you buy one low, then sell your ugly set for melt locally. it shoudl be a near break even deal in the end.