hmdrake Wrote: Avoiding gamer terminology is one of the reasons I'd say avoid Ultra Rare. I'd use Exceedingly Rare or something similar to avoid the implications.

That's a point, although I don't know if I can avoid that. "Exceedingly" is a bit long and I'm not entirely comfortable with that.
I'm still considering a variant of the color idea suggestion from above, one with a number of modification to bring other points across, but if that fails I'll probably keep ultra rare.