This is actually one of the best use of Gems I've found:
Sandslash @ Ground Gem
4 HP / 252 Atk / 252 Spe
Strategy is simple, switch it in against something slow and weak (Forretress, Deoxys-D) and Sword Dance up. Here's some calcs.
+2 vs Standard Jellicent: (94.54 - 111.16%) Guaranteed OHKO with SR.
+2 vs Gengar: (146.56 - 173.28%) Guaranteed OHKO
+2 Burned vs Sableye: (73.35 - 86.84%) Ok, you lose there, unless you put in around 15% prior damage.
+2 vs Dusclops: (71.83 - 84.5%) Guaranteed 2HKO
You get the idea. Apart from Sableye who survives thanks to Priority Will-o-Wisp, every Ghost type loses. Which means that, assuming Sand is up and with 1 Sword Dance, you can guarantee a spin. Either switch-in, SD, then Rapid Spin or kill their spinblocker, *then* Rapid Spin.
After Spinning, you're still left with something that outspeeds everything up to Scarf Mienshao, with fairly good bulk, power and STAB Earthquake.
L'ennui est ma Muse.