1 Pound Coin with Strange Brown Colour [gelöst]

Hello, I have this 1989 1 pound coin that is strangely coloured dark brown. I have never seen coins so badly toned like this. Could this be due to heavy oxidation or could this be a mint error with a coin blank of the wrong metal? The edges are also completely chocolate coloured, but some scratches do reveal that the coin is the normal gold colour inside. Sorry for my bad english and any help would be appreciated. Thank you.

an ordinary pound, it seems to me, visited in an unfavorable environment ...

We see this a lot in the UK when the old £1 is left out to the element's not worth a lot if you want to Keep it you can either leave it as it is or Clean it as it wont decrease its value ay all.

Thanks a lot