Barkeeper's Friend? It's good for metal marks and I bet that's what you've got there. Belt buckle! :)

Acetone on a rag; if that doesn't work, then acetone on a Gray Scotch Brite.

I agree with trying barkeep's friend, if that doesn't work, acetone. check before you can use a scrubby like scotch brite, it might be considered too abrasive for fireclay. not sure.

I use barkeep's friend and a plastic scrubby cloth (like this one) on my enamel cookware and it gets everything out without damaging it.

be careful what you use magic erasers on as well. it's basically like super fine grit sandpaper. probably wouldn't damage that sink, but you'll want to be cautious what things you do use it for.

Try rubbing gently with a damp rag sprinkled with Bon Ami powder.

Try Astonish oven cookware and cleaner. My Shaw fireclay sink was marked also when the countertop was installed. I contacted Rohl and this is what they recommended for their fireclay sink. You can buy on amazon. Use it with a green scrubby. Good luck.

Did you ever get the stain out? This same thing happened to me I think with my husbands wedding ring...

Here is pic.