It is stained mottled green and brown at the bottom of the bowl (about 1/3 of the bowl is discolored); I believe that the ceramic has been eaten away and is quite pitted. There's really nothing to be done about it short of buying a new toilet, which isn't in the cards for our rental. I've tried to clean it, BF has tried, our maintenance guy has tried, and my mother (who could clean the spots off a leopard, she's that amazing at cleaning) has tried ...
If you didn't know any better, you might think it was caused by mold or a lack of proper, frequent flushing on our part, I think. There is also staining or rust around the base of the toilet and the bolts, which have cracked covers, but you wouldn't see that unless you really peered down to look. The management of the complex is new and great, but it's clear that the previous maintenance folks were awful. They painted over dingy tile and tub stains rather than replace or really clean them, for example.
I'm pretty sure the last tenant (or last several) weren't too observant of regular cleaning procedures, which didn't help.