Yep, take Todd's advice. The Works is a great acidic toilet bowl cleaner, readily available, and not pricey. I've never had much luck with those pumice stones. I have clients with some bad lines in their bowls from water that ran over time, and tried them with no luck. Of course, no cleaner will touch them either. Maybe they're just etched.
I took some deposits out of a kitchen sink once that nothing, and I mean nothing, would touch. I used Bridgepoint's T-Rust. I let it soak for 5, then gave it a light scrub with a scotch-brite pad (wear gloves!). Amazing!
It contains ammonium hydrogen fluoride, a cousin to hydrofluoric acid, but not as dangerous.
Use an acid bowl cleaner that foams and will cling to the bowl and work for a few minutes. T-Rust works well also. Home Pro has a great little tool called the Bowl Boss that works similar to a pumice stick but has a handle. This makes it much nicer to use. https://www.hpbyd.com/order.asp
In the past I've always used C.L.R. and if that just wasn't working . Go get the big gun (Muriatic acid) and apply lightly and fast and run from the fumes. It would be wise to get some air circulating in the area to get the fumes out of there as fast as possible.