1999 P Dime...something Missing?....could It Be Improperly Annealed

This was an outdoor find. It is affected with a coating on the coin. Just a spendable coin. The color will return. It is not a missing clad coin.

oh and I forgot to mention the coin is actually very dark/black on both sides

People spend the common coins they find metal detecting all of the time.

I dont metal detect lol...all coins and bills I get are from change I get at a store. Matter of fact I got 2 more crisp consecutive star notes, a 2021 penny, 2021 dime, and a 2021 nickel today from change at Dollar General. To be honest for some reason I get a lot of interesting coins and bills from there.

Never said you did, I stated people who metal detect spend the common coins they find and then other people will receive them in change, thus getting an environmentally toned coin is very possible.

I do metal detect and that is what this coin is. It happened well before you got it, you can see where the high points have worn to a more normal appearance from circulation
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@NaNa7 Might try one or two experiments. First place one or more clad or Cupronickel / copper-nickel (CuNi) on top of some soil. You can, but I found no need to bury underground. Can be in an grassy area, asphalt, fence post or concrete as well. Just expose to some sunlight and morning dew.

The toning wont visibly appear over night, but will start to evolve over days, more over weeks, even more over months and years. Toning rates differs pending the type of environment, soil type, acidity or fertilizer presents.

Second experiment I found useful more than once. Note I did not have a metal detector, but did adopt a one year old (red nosed) American Pit Bull Terrier that I named Halo.

Day one (Halo's 1st find) after I made several pleas with him to stop, he continued against my wishes and decided to dig an eight inch deep hole alongside my driveway. After he was finished (or lost interest). I looked to see what he was after and found a 1990 dime was uncovered at the bottom.

Outside it being completely toned, it was in pristine condition. Note after a quick rub against my cotton t-shirt, revealed no signs of pitting nor any dirt remnants. It was fully toned a bit darker, but similar to your 1999 example.

Note if placed back into circulation, the toning will start wearing off (rim and high points first). Thanks, Doug.

PS: Yes without question I'd let Halo sniff and dig there after. He almost always found something be it coins, bills, yes even some bones. Thank goodness no bones seemed human, but if he had lived longer, I've no doubt they too would've eventually been dug up.

Sorry for yours eyes. This coin was 25% nickel and clad copper. The colors you have is due to long term humidity. I am sorry to say you this , but this it is the chemistry. Look at the coin, only low part of was affected, not others. Sorry no error on this coin.

Hope you will find one good.