I would think you could use phototransistors for your sensors, they work by emitting infrared light, and then reading how much of the light is reflected back. So it will be able to detect if an object is there or not. It will also read a different value based on the color of the stone because different colors reflect the IR light better. This is an example of what I mean.
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Guess i was a little behind on that post... However I do think you could tell the difference between the colors depending on the finish of the stone ie Rough or Glossy.