Yes. Pretty normal and natural process, it's from oxygen and chemicals in the air reacting with the metal. Not much you can do about it. You can prevent your other coins from toning by storing them properly (temp controlled room etc) and vacuum sealing them. Still not guaranteed 100% effective. Toning can actually increase the value of certain types of coins so it may be work in your favour. Any pics of it?
I have a '91 proof with a lot of toning around the rim. It's graded PF69 so it doesn't really bother me but I guess it would some folk, like the guy who sold it to me! It was my first purchase in Pandas so it has a special place in my collection. I'll try to get a photo of it up soon for you Tas
I've heard a few of the 2001 have some toning.. put it this way I'd rather have a bit of toning than giant white milk spots like my Maples have!! Stock bullion those coins. Hmmm I forgot how much I love the old skool temple there, that bold, clean design of the early Pandas got me hooked.
I would say toning has a lot to do with how the previous owner stored the coin. A lot of old timer collectors put Silver coin in Dansco Album which contains acid and other chemicals. After that, the toning will show up sooner or later. I bought 2001-2009 Panda BU coins from an Ebay seller in 2010, who stored all the coins in a Dansco album. I took all of the coins out as soon as I got them (no toning shown at that time), and put them in capsules. now after 4 years, most of them have developed toning at the edge. I also bought the entire set of Kooks from the same seller who sold them in a dansco album too, same thing happened. Although 1 got toning as soon as I got them, so I returned it. And now after 4 years or so, most of kooks also got toning at the edge. Even though I got excellent deal in buying those, but now all the toning ruined the most of the value out of these coins. And at the same time, I bought 10 2010 Panda and also some 2010 kooks from individual sellers, who kept the coins in the original Mint capsules. NONE of those got any toning. This tells, it has nothing to do with my storage. It has everything to do with the previous owners putting them in Dansco album that contains Acid and other chemicals