Your granite slabs may look impenetrable, but they are capable of becoming stained. Natural stone is quite porous. When spilled liquid gets into granite's pores it occasionally leaves a stain behind as it evaporates. Rust and other stains are relatively easy to remove. But scrubbing the surface of the granite is not likely to be effective. The cleaner must be able to access the granite's pores where the stain resides.
Clean the stained area with soap and water. Rinse it clean.
Mix a commercial rust remover (like Iron Out) with flour until you create a poultice the consistency of peanut butter. The amount of poultice that you need will depend on the size of the rust stain to be treated.
Cover the stained area and 1/4 inch beyond it with the paste. Make the layer roughly 1/4 inch thick. Thicker poultices do not clean any more effectively and they take longer to dry.
Cover the poultice with plastic wrap and tape the plastic wrap down at the edges.
Leave the poultice to sit for 24 hours, then lift the plastic to see if it has dried. If it has not, leave it to sit for another 12 hours and check it again.
Scrape the dried paste off of the granite with your hands, then rinse the area clean with water.
Re-apply the poultice if the rust stain is not completely gone.