A boost die to attack rolls, a setback die to defense or deflect, or as mouthymerc pointed out superior quality for its builder.
Except that's the sort of thing that should be provided by Talents, reflecting the person's level of skill with the weapon instead of whatever funky gem the person managed to shoehorn in.
And Superior quality with it's free Advantage is rather dangerous given that lightsabers have a Crit Rating of 1, which makes the Jedi more likely to inflict a critical injury, particularly with the Breach quality ensuring that damage gets through.
If anything, the Superior quality should be held in reserve as something gained by a high-level talent, particularly as the lightsaber is already deadly enough without it. And remember that one of FFG's stated design goals was to have Force & Destiny PCs be playable right alongside EotE and AoR PCs, so providing too many bonuses for lightsaber modification is going to skew things, especially if the suspected Jedi career allows the option of beginning the game with a lightsaber.
Edited by Donovan Morningfire