The biggest difference is actually not the "black hole" effect, which can be achieved using prof's xtal. The gen xtal has a more silvery reflection on certain angle, while on other angle is VERY clear. Also, the edge of the gen xtal is polished slightly differently - shinier & nicer. However, most people may not appreciate the subtle differences, but nonetheless they are what contributes to the "gen" aura that many of us long-timers here do appreciate.