Turn off the water to the toilet and flush so that no water is in the bowl. Add CLR (available at most stores) and let it sit for the rec commended time. Brush to clean. Turn the water back on and...
That is an acidic etch. You don't remove it. A pro will repolish for a hefty fee, look online for granite restoration companies.
Granite has to be treated carefully, allow nothing but granite...
You have to use some of your coke at home. Before you sleep, pour some of your coke on the stain and you can also add some bleach and let it be there overnight and rinse it off the next day.
