I don't mind wearing some ice although I would never go as far as Mr T!
I just can't work out why they are green gems?????

Green gems seem to suggest or invoke the green eyed monster (jealously): http://en.wikipedia.org/wiki/Jealousy.
Also infamous in the east as jade, which is typically a grayish shade of green, when made into Chinese jade dragons, e.g: http://www.mercurytreasures.com/jachdr.html.

By contrast red gems seem to suggest life, family and are also popular when cut in shape of hearts. Fire engines, hydrants and extinguishers are also usually red.

The only obvious thing I could think of where green is go and red is stop are traffic lights.

If they are gems then:
+ Good Rep = Red gems with different shades as rep grows.
0 Rep = A gray gem
- Bad Rep = A green gem (green eyed monster), with different shades as rep gets worse.
Alternatively bad rep members don't deserve any bling and should therefore lose their gems altogether and have an icon which is not a gem instead e.g skull.

Otherwise if they are lights then:
+ Good Rep = Green lights or Go!
0 Rep = Amber light.
- Negative Rep = Red Light or Stop!

So are they reputation gems or lights and why?