What's up with this penny?

So I found this penny amongst my large collection of random pocket change. I noticed it when I was searching through the coins for anything worth keeping, before I turned them into the bank. I found this penny, it's got such a odd color. It's kind of golden. It's a 1999 d penny. So what's up with it? I've got pics with a regular penny and a dime in the shot for comparison.

 

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Just environmental toning is the only thing i can think of.

 

Hmmm well either way, I like it, I think I'll keep it.

 

Someone experimenting with electroplating perhaps?

 

I have produced the same effect by heating zinc cents.

 

Yeah it's the one on the right.

 

Sometimes when the planchets are plated some of them can be overdone and come out looking more like brass then copper.

 

Would you mind explaining what that means in terms of value and collectablility? I'm a bit of a noob. I'm gonna keep it either way, I like it. But I'm just curious.