Lord of the Toilet Rings

Does anyone know how to get this ring to go away from my downstairs, rarely used toilet?? The same thing happened to the extra toilet when I lived in my condo, but a pumas stone cleared it right up!! I tried the pumas stone today...the only thing that happened: I broke a sweat while kneeling on the bathroom floor, hovering over the toilet, with my hand completely immersed in toilet water topped off by a ring that wouldn't go away no matter how much I swore at it!!