This post was published long before the Mists of Pandara Expansion.
The tips and techniques explained here may be outdated.
Gem of a Deal
30 Stone Keeper Shards -> 2k honor
3 weekly quests in Wintergrasp -> ~10k honor
9 Wintergrasp tokens -> 10k honor
In Stormwind at JC vendor:
10k honor -> 1 epic gem
In Dalaran at Heroic badge vendor:
20 heroic badges -> 1 pure epic gem (red, yellow, blue)
10 heroic badges -> 1 hybrid epic gem (orange, purple, green)
Get epic gems! Grimr says! There’s no excuse not to.