[Baalz]

There's a die roll involved in the dispel, it's described in the manual. The amount of gems used generally overwhelms the relatively small amount of variation from the die roll, but when you're talking about min or max gem usage die roll is what matters. Also, don't forget that caster level factors in, so a level 9/10 pretender putting up a 999 gem spell is pretty tough nut to crack. In extreme (large game) cases it can even be worthwhile to empower up even higher if you're putting up a game winning global that is certain to have a big dispel cast at it.