I think it's called a struck-through error, when an object is between the coin and the die, leaving an imprint. Did you see an object sticking to the coin when you first saw it? If not, it will be an un-retained, which is a very common error. If there is, then this coin is now a very valuable collectible. I think you've got your hands on a valuable coin, it's in good condition and the error gives it value. Here's where I think you can find it, if you haven't: N#3167 Good luck on your coin!
To me its post mint damage as we see no interruption of the beading Crazy Glue or some type of clear polymer material. Post mint type circumstance. In cases like this let it soak in acetone or alcohol (IPA) for 24h and report back.
Looks more like a metal fragment fell onto the blank and got fused with it after minting (if it's delamination it doesn't seem to come from this planchet maybe a catastrophic minting event happened shortly before this one was struck and some remnant landed on it). I thought struck-through errors don't have the object they struck through on them maybe it's called retained struck-through or similar..
Belgisch Congo • 10 Centimes - Albert I
