Gem vendors

gems can be purchased from the following vendors for
 * in Honor Hold
 * in Thrallmar
 * on Aldor Rise, Shattrath City - checked, as at 28Jan2010, she does NOT sell gems, she sells designs.
 * on Scryer's Tier, Shattrath City
 * in Stormspire

Some gems can be purchased from  in very limited-supply, for  each.

After phase 4 of the Shattered Sun Offensive, Shaani starts to sell all six types of Burning Crusade epic gems for 15 Badge of Justice each at the alchemy lab.