I haven't been successful posting photos. I decided to do this first in a basement, where the walls were firred out. The jambs must be 10 1/2" wide. I thought there was no way to use anything but plywood on something that wide, and they always get so dirty and beat up.
I brought out the tile the thickness of the window casing. In this case, it was the real cheap ranch style oak window casing, 2 1/4" wide. It is about 3/16" thick at the thin side, so I had the tile come out proud of the sheetrock by 3/16" and it is flush with the trim installed.
Just be sure to use porcelain (through and through), so the edges look nice. You'll find on normal windows (less than 4" extensions) you can use up your scraps.
Of course you could also use granite, marble, whatever. You could also use one-piece sills as is common in other parts of the world.