Interesting question! I don't know the actual reason, but if I had to wager a guess I'd say that it's due to the different types of doping of the silicon.
Some searching did indicate that is what gives some types of gemstones their colour (rubies for instance receive their red colour from chromium impurities distributed throughout the lattice structure http://en.wikipedia.org/wiki/Dopant#Examples). It seems plausible, but there is probably a more accurate answer out there.
Some searching did indicate that is what gives some types of gemstones their colour (rubies for instance receive their red colour from chromium impurities distributed throughout the lattice structure http://en.wikipedia.org/wiki/Dopant#Examples). It seems plausible, but there is probably a more accurate answer out there.