Wash the piece in dish soap and water. Scrub the spot with a nylon dish scrubber to see if the stain comes off. Rust should lift easily from a uniform finish in good condition.
Spray some lemon juice or vinegar on the stain. Let the acid sit for a minute and then sprinkle it with salt to function as a gentle abrasive while you gently rub with a cloth or your nylon scrubber.
Make a paste of washing soda – also sold as sodium carbonate – and hot water. Apply the paste to the stain with a damp, clean cloth and let the paste sit for a minute or two before scrubbing and rinsing. If the stain starts to fade, reapply the washing soda paste and let it sit for a few more minutes.
Spray the stain with oxygen bleach or make a paste of the powdered version and rub on with a damp cloth. Do not allow the bleach to sit for longer than one or two minutes before rinsing to see if the spot fades and then reapply as needed.
Scrub the stoneware with a soft abrasive product that contains bleach. Use a cloth – your nylon scrubber combined with the abrasive may scratch the dish.
Rinse the dish well after removing the stain. Wash the stoneware with dish soap before using it on the table again.