Prismatic shard has always caught my eye. It's a busy design, aesthetically, but it's so wonderful when you finally find one and stare at it like you won the lottery. :3
Prismatic shard for me, the combination of the pretty visuals and that feeling of accomplishment is just perfect. The clothing is also pretty good. Aquamarine close second though.
The gems are all pretty but everytime I've dug up a diamond I always pause for a sec and stare at the little graphic on the bottom left when it goes into my inventory. The diamond with the purple background is gorgeous to me.