Have you tried a pumis stone? It’s a small grey stick made from volcanic ash , it does not hurt the toilets. Have you tried clr, cleaning vinager?
I haven't tried this, but it is worth a try.. https://www.littlethings.com/cleaning-the-toilet-kool-aid/" rel="nofollow noopener" target="_blank">.https://www.littlethings.com/cleaning-the-toilet-kool-aid/
Most likely it's a build-up of minerals from your water. Use a toilet brush to push out most of the water in the bowl.Take a cloth or paper towels, wet them with vinegar water. I use a mix of equal parts of vinegar and water with a few drops of Dawn. Put the cloth or paper towels on the ring and let it soak for 30 mins or so. Then wipe it off and rinse by flushing the toilet. If the build-up is significant, you might need to repeat the process. Wishing you the best.
At any grocery store near the toilet cleaning products is a scrubbing stone specifically for porcelain. Scrubs off all mineral deposits easily without damaging the porcelain. It sounds like it's scratching it but it really doesn't!!
Try salt , a generous amount and a fine bristle brush or sponge. Should do the trick works on rust also
Try table salt and a brush or cloth smidgen of h2o
remove water, wet paper towels with vinegar and put on ring let sit on it the longer the better then make paste out of dawn and baking soda scrub on ring it'll be really fizzy with vinegar; If that doesn't work use straight CLR on the ring remove water put ClR on paper towels let sit on ring srcub well with stiff scrubber pad..