I think its hard water brown line around toilet bowl.
After cleaning the toilet well, I roll up a couple of paper towels, drenched with bleach and lay the towels around the brown ring area. Let the towels remain for as long as you can and then retrieve them and toss into trash. DO NOT flush it down the stool! I do this about every 3 weeks to keep the stain away.
You can scrub with a pumice stone to remove. Do you have hard water with lime? Maintenance to keep it from coming back will depend on your water content. I drop a denture cleaning table in my bowl sometimes. A cap full of chlorine bleach when the toilet isn't going to be used regularly (guest bath).
Bleach removes many stains but there are many others like baking soda and vinegar and Efferdent that is used to clean dentures.
Have you tried toilet bowl cleaner and a scrub brush? If not that then maybe some bleach
I have tried most of those things and nothing. The only thing i havent tried is Denture tablets!
a cleaning supply house carries a sand paper product that does quite nicely. Use rubber gloves and be sure to use gentle pressure.
The Works toilet bowl cleaner has always been my go to for hard water stains. Their original one. Very thin like water. This stuff has melted rust stains right off the inside bowl when no other cleaner has touched them and I didn’t even use a brush yet! I even used it on brick that a kid sprayed with orange flourescent paint after all the paint removal stuff didn’t touch it. Also rust on that same brick from faucet overspray. It also melted carpet went I spilt it so be careful! I currently just got a pumice stone for that stubborn line which might be permanent etch? Haven’t tried it yet.