Emerald Ore

Emerald Ore is a 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. The ore spawns anywhere between levels 4 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, and these might be more common than veins of diamond, because diamond can attempt to spawn in bedrock (and thus fail to spawn). Because the veins are smaller, however, emerald may be slightly rarer than diamond anyhow.

History
Starting from Survival Test, Diamond was apparently going to be called Emerald.

Emerald Ore was first seen in the preview screenshot from Jeb and had a red texture; it was later released in snapshot 12w21a.

In snapshot 12w21b the texture of the ore was changed from red-dotted to green-dotted.

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.