Emerald Ore is one of the rarest Blocks in Minecraft. It first appeared in 12w21a and was finally added in the 1.3.1 update.
Emerald Ore was added in the 0.9.0 Update. It is one of the eight Ores to be added to the game after Coal Ore, Iron Ore, Gold Ore, Redstone Ore, Lapis Lazuli Ore, Diamond Ore, and Nether Quartz Ore.