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 certain biomes, at the moment it's confirmed to be generated below extreme hills and jungle biomes, and not been generated below desert and ocean. The ore seems to spawn anywhere between levels 1 and 30 - it's worth noting that this is similar to the range of gold and lapis lazuli.

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

 * Emerald Ore is approximately 1/40 as common as Diamond Ore(40 times more rare then Diamond)(174 Emerald Ore in 2382 chunks, compared to 6637 Diamond Ore).
 * 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.