Pumice stones are volcanic rocks with a porous and abrasive texture, which can remove mineral deposits, hard water stains, rust, and even stubborn limescale from your toilet bowl.
Wet the stone and gently scrub the stained areas. While you scrub, the stone will begin to break down and deposits will shed into the bowl, which can be safely flushed away.
The risk of scratching the toilet bowl is minimal, but remember to test the stone in an inconspicuous area first to ensure compatibility with your toilet bowl.