Awesome deck, I might include Primal Surge because it can help you get a bunch of your big beefy creatures on board. I might also consider Terastodon or Woodfall Primus as both big creatures and ways for extra removal.

I might also switch out Hall of Gemstone for Caged Sun but upvote from me! Looks like fun!

Primal Surge is a good option, but ultimately I've gone with Genesis Wave and Lurking Predators to fill its role. Primal Surge usually costs as much as Genesis Wave, and it has the possibility of going bad on the first card. It's a 1 in 10 chance, but it's happened enough that Genesis Wave consistently outperforms it. Terastodon definitely could use a place in the deck, but I've gone with Wave of Vitriol for a more immediate effect, given my low graveyard recurrence. I think you might be confusing what Hall of Gemstone is in there for--it's not intended for mana generation, but for mana control. It locks down multi-colored opponents and prevents anything but green spells from being played on my turn, so there's effectively no counterspells.