Emerald Ore

Emerald Ore is an extremely rare block that drops Emeralds when mined with an Iron Pickaxe or better; it does not require smelting. It is only generated in Extreme Hills and Extreme Hills Edge biomes, and does not generate at all in other biomes. Note that it can still occur in River biomes if the river cuts through Extreme Hills. The ore spawns anywhere between layers 2 and 31, meaning it can occur as high up as gold can, and is less likely to occur at the bottom 4 layers of bedrock. Veins of it are as large as veins of Lapis Lazuli.

Ore distribution
Emerald Ore was only found in Extreme Hills or Extreme Hills Edge biomes until Jeb removed it. An analysis of a 500x500 Extreme Hills Biome with MCedit gave the following ore distributions:

The Graphs show that the distribution of all ores except for Lapis Lazuli is even in a certain range and falls off sharply at the borders. While the total amount of emerald is with 3386, even higher than the amount of Diamonds (2997), the ore is spread out over double the height range, meaning that at a certain level the chance for finding an ore is just about half of the chance of finding a diamond ore, effectively making it twice as rare when strip mining.

History
Starting from Indev, Diamond was originally named Emerald.

On May 21, 2012, Jeb released a screenshot of himself testing the trading system. At this time, what would become Emerald Ore was Ruby Ore.

In Snapshot 12w21a, the item was changed to an Emerald Ore. The texture for Emerald Ore remained unchanged, as Jeb forgot to commit the new texture in the Snapshot. As this item was not announced before the Snapshot was released, most translations call Emerald "Diamond."

In Snapshot 12w21b, the texture was corrected from to. Translations of Emerald as Diamond were also corrected.

Jeb stated on Twitter that he would not be removing Emerald Ore for the 1.3 update, but he confirmed that he would leave it at its current rarity.

Trivia

 * Ruby and Ruby Ore can be found in the lang files, which indicates that the gem and ore were originally supposed to be rubies. This is supported by the fact that the first screenshot of the trading system released by Jeb featured red gems rather than green ones.
 * Lapis Lazuli and Emeralds are the only ore blocks with unique textures.
 * Emerald is the only biome-specific ore.
 * Although there is more Emerald Ore than Diamond Ore in Extreme Hills biomes, it is spread over twice the depth, which may make it harder to find.