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 levels 1 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
As mentioned above, Emerald Ore is only found in Extreme Hills or Extreme Hills Edge Biomes. 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 Survival Test, Diamond was originally named Emerald.
 * What would become Emerald Ore was first seen in the preview screenshot from Jeb. At this time, it had a red texture. This version was released in Snapshot 12w21a by accident.
 * In Snapshot 12w21b, the texture of the ore was corrected to better match the item it drops, being green instead of red.

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 a different pattern from the other ores.
 * Emerald is the only biome-specific ore.