This is Rebecca Sugar's OFFICIAL classification of the gem's power level. This list is disregarding FUSIONS who have their own tiers.

S: Steven Diamond Universe, White Diamond

A+: Yellow Diamond

A: Blue Diamond

B+: Pink Diamond

B: Lapis, Aquamarine

C: Jasper, Emerald, Hessonite

C: Holly Blue Agate, Topaz

D+: Amethyst, Pearl, Bismuth

D: Ruby

F: Sapphire, Peridot