Mystery black mark on bezel of silver iPhone X

Hello - a mysterious black mark has suddenly appeared on the bezel of my silver iPhone X. Specifically it is found in the upper left hand corner - almost level with the lower camera lens - right on the edge of the bezel where this abuts the back of the phone. Squinting at it reveals that it is found on the thin band white band that runs around the outer edge. It's either quite dark or black and looks too neat to be a scratch or chip in the case - it's almost a half circle. In any case I had understood that the bezel on the silver iPhone X was made from stainless steel, which would not reveal a secondary colour if chipped? Having said though, I don't know what the white band is made of (or even what it's for)...If it were unambiguously a scratch in the stainless steel, I would buy some metal polish and get to work. As it is, I don't know what to do and would welcome any suggestions or input. My first thought was that it was a stain or a bit of debris that had somehow got in the barely paper thin gap between the bezel and the back plate, but now I'm not so sure.

Yes, I know that cosmetic blemishes like this do not affect the functioning of the phone, but a) it's unsightly on a device designed to look good; b) it will reduce the resale value of the phone if I can't fix it somehow and c) it's annoying because it just appeared and I don't what is or what caused it.

I have attached a couple of okay-but-not-great pictures: it's difficult to photograph something quite small like this without a proper macro camera which I don't have unfortunately