wow,
thanks everyone for replies, especially fabiopigi, for all the effort.
i think i like the idea with fiberoptics and a webcam the most. it's true, i'd do most of processing in computer, so arduino is not totally necessary. ah, to explain a bit more, the basic idea is to make a kind of self-contained A/V instrument from a GO game. ideally it would produce audio (drone, glitches, noise, textures) and two VGA video outputs for projection while two (or even just one) would play the game. the audio/video would be produced with computer. i'm quite fluent with Pure Data, so that's my home programming enviro. this is meant to be some kind of performance.
i'll look into anyway how to work with fiberoptics. i would imagine to fix the cables into a mini grid at the end and try to read it with a webcam or directly with an image sensor. any thoughts on that? also, how would be possible to trigger the led on and off without a complicated circuitry at the point of placing the stone? this would be somehow mechanical i would presume. maybe the leds should be on all the time but shielded from the fiber, like fabiopagi has drew.