Try something like Lime-away. It sounds like some good water deposits. Or at least you'll feel better if you think of it as water deposits. :)

Our plumber recommended using a pumice stone for cleaning rings and mineral build up. They make ones for cleaning toilets. One brand is called Pumie. I bought one at a local hardware store and it worked great.

Make sure you get up into the holes where the water comes out under the rim of the bowl. The lime deposits can clog the holes.

plunge out what ever water is setting in it

then pour a gal (Or 2) of pure vinegar into it and let it set a day or 2. brush and flush and see what goes w/it.

I've done it here in a very old unused avocado green toilet. It left an edge higher up to be cleaned (the water line). wet a few layers of tp with vinegar and cover that. I put some vinegar in a squirt bottle and rewet it continually. It's better to remember to keep it rewetted (is that a word?). I was lax at the remembering part... and it will smell of vinegar to high heaven!

then put on plastic gloves and use a SOS type pad on it around the edges.

deep down in the hole will be harder...

you might still have to scrape a bit at crust layer down in the hole. after setting in the vinegar it should be easier tho.

if needed, repeat and repeat...

I gotta say I was actually shocked that the center area (that was filled with the vinegar) cleaned out slicker'n a whistle! (ugh - maybe that's bad phrasing in this case).