Common gems can be purchased from the following vendors for 2g

Color Name Effect
UI-EmptySocket-Red.png Red  [Bold Tourmaline] +4 Strength
 [Delicate Tourmaline] +4 Agility
 [Brilliant Tourmaline] +4 Intellect
UI-EmptySocket-Yellow.png Yellow  [Smooth Amber] +4 Critical
 [Subtle Amber] +4 Dodge
UI-EmptySocket-Blue.png Blue  [Rigid Zircon] +4 Hit Rating
 [Solid Zircon] +6 Stamina
 [Sparkling Zircon] +4 Spirit

Some uncommon gems can be purchased from Neutral Dealer Senzik in very limited-supply, for 4g each.

Color Name Effect(s)
UI-EmptySocket-Red.png Red  [Bold Blood Garnet] +6 Strength
 [Bright Blood Garnet] +12 Attack Power
 [Delicate Blood Garnet] +6 Agility
 [Runed Blood Garnet] +7 Spell Damage
 [Teardrop Blood Garnet] +13 Healing
 [Brilliant Blood Garnet] +6 Intellect
UI-EmptySocket-Yellow.png Yellow  [Gleaming Golden Draenite] +6 Spell Critical
 [Smooth Golden Draenite] +6 Critical
 [Subtle Golden Draenite] +6 Defense
UI-EmptySocket-Blue.png Blue  [Lustrous Azure Moonstone] 2 Mana every 5 seconds
 [Rigid Azure Moonstone] +6 Hit Rating
 [Solid Azure Moonstone] +9 Stamina
 [Sparkling Azure Moonstone] +6 Spirit
 [Stormy Azure Moonstone] +8 Spell Penetration
UI-EmptySocket-Red.pngUI-EmptySocket-Yellow.png Orange  [Inscribed Flame Spessarite] +3 Critical +3 Strength
 [Reckless Flame Spessarite] +7 Healing +3 Intellect
 [Potent Flame Spessarite] +3 Spell Critical +4 Spell Damage
UI-EmptySocket-Yellow.pngUI-EmptySocket-Blue.png Green  [Dazzling Deep Peridot] 1 Mana every 5 seconds +3 Intellect
 [Regal Deep Peridot] +3 Defense +4 Stamina
 [Jagged Deep Peridot] +3 Critical +4 Stamina
 [Radiant Deep Peridot] +3 Spell Critical +4 Spell Penetration
UI-EmptySocket-Blue.pngUI-EmptySocket-Red.png Purple  [Timeless Shadow Draenite] +4 Spell Damage +4 Stamina
 [Purified Shadow Draenite] +7 Healing 1 Mana every 5 seconds
 [Shifting Shadow Draenite] +3 Agility +4 Stamina
 [Sovereign Shadow Draenite] +3 Strength +4 Stamina
 [Glinting Shadow Draenite] +3 Hit Rating +3 Agility

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.