1982 British 5 Pence Proof

Not sure this is under the right forum title so if not hopefully a moderator can move it for me.

I have a 1982 british proof set that I was considering repricing and selling. When I took it out of the package to carefully check it out I noticed it is beginning to tone. Most of the coins are toning in the normal way (outside ring towards the middle) but the 5 pence coin is quite strange looking.

It has toned halfway up (from the bottom to top) I takes on a blue color when tilted. I was just wondering if this is a common occurence in the proof sets?

I am going to keep trying to get photos but so far the plastic case has effectively thwarted any attempt at a proper image.

thanks
Malissa