AFAIK No games actually "sort" balls, Pretty sure Gold Ball has two separate ball feed positions and the gold finish on the ball is just for show. The rest of them just detect when the special ceramic ball comes into play.

For materials sensing your options are a reed switch or some sort of hall effect or eddy sensor, or a switch closure like it sounds like Avatar LE might use, not familiar with it. If the ball is stationary than an albedo sort or color detection would also probably be possible with an RGB led and one or more cheap phototransistors.

Another option: If you're already going to be getting custom ceramic balls or something in any sort of quantity, it's possible that you could get them with a passive RFID embedded in them. No idea what that would cost you. With a low power antenna you could limit the effective range to a couple inches. That would be sweet.