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

Color Name Effect
UI-EmptySocket-Red Red [Bold Tourmaline] +4 Strength
[Bright Tourmaline] +8 Attack Power
[Delicate Tourmaline] +4 Agility
[Runed Tourmaline] +5 Spell Damage
[Teardrop Tourmaline] +9 Healing
UI-EmptySocket-Yellow Yellow [Brilliant Amber] +4 Intellect
[Gleaming Amber] +4 Spell Critical
[Rigid Amber] +4 Hit Rating
[Smooth Amber] +4 Critical
[Thick Amber] +4 Defense
UI-EmptySocket-Blue Blue [Lustrous Zircon] 1 Mana every 5 seconds
[Solid Zircon] +6 Stamina
[Sparkling Zircon] +4 Spirit

Some uncommon gems can be purchased from Neutral 15 Dealer Senzik in very limited-supply, for 4Gold each.

Color Name Effect(s)
UI-EmptySocket-Red 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
UI-EmptySocket-Yellow Yellow [Brilliant Golden Draenite] +6 Intellect
[Gleaming Golden Draenite] +6 Spell Critical
[Rigid Golden Draenite] +6 Hit Rating
[Smooth Golden Draenite] +6 Critical
[Thick Golden Draenite] +6 Defense
UI-EmptySocket-Blue Blue [Lustrous Azure Moonstone] 2 Mana every 5 seconds
[Solid Azure Moonstone] +9 Stamina
[Sparkling Azure Moonstone] +6 Spirit
[Stormy Azure Moonstone] +8 Spell Penetration
UI-EmptySocket-RedUI-EmptySocket-Yellow Orange [Glinting Flame Spessarite] +3 Hit Rating +3 Agility
[Inscribed Flame Spessarite] +3 Critical +3 Strength
[Luminous Flame Spessarite] +7 Healing +3 Intellect
[Potent Flame Spessarite] +3 Spell Critical +4 Spell Damage
UI-EmptySocket-YellowUI-EmptySocket-Blue Green [Dazzling Deep Peridot] 1 Mana every 5 seconds +3 Intellect
[Enduring Deep Peridot] +3 Defense +4 Stamina
[Jagged Deep Peridot] +3 Critical +4 Stamina
[Radiant Deep Peridot] +3 Spell Critical +4 Spell Penetration
UI-EmptySocket-BlueUI-EmptySocket-Red Purple [Glowing Shadow Draenite] +4 Spell Damage +4 Stamina
[Royal Shadow Draenite] +7 Healing 1 Mana every 5 seconds
[Shifting Shadow Draenite] +3 Agility +4 Stamina
[Sovereign Shadow Draenite] +3 Strength +4 Stamina

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.