Emerald Ore

Emerald ore is a rare mineral block that drops emeralds when mined with a diamond or iron pickaxe. Emerald ore does not require smelting. It drops one experience orb when mined, unless mined by an iron or diamond pickaxe with the silk touch enchantment. It is only generated in Extreme Hills or 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. As of Snapshot 12w22a, it does not generate in veins as most ores do: rather, 3-8 blocks of stone in a chunk may be replaced by emerald ore. This means it is often encountered as a single block, unlike most ores, and tends to be very scattered.

Ore distribution
Emerald ore is only found in Extreme Hills or Extreme Hills Edge biomes. It attempts to generate 3-8 times per each chunk, generating anywhere between layers 4 and 31, meaning it can occur as high up as gold can. It will only generate in locations which would have otherwise been stone - it is thus possible for as few as two to zero emeralds to generate in a chunk. Found as individual blocks, rather than veins, it can be encountered more commonly than gold, which has a similar range.

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. The previous ruby texture remains in the items.png file above the emerald texture. Translations of emerald as diamond were also corrected.

In Snapshot 12w21b, 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 was just about half of the chance of finding a diamond ore, effectively making it twice as rare when tunnel mining.

For Snapshot 12w22a, emerald ore would have no longer generated naturally. This was due to the large number of complaints by users about emerald ore's rarity, despite Jeb's wishes for it to only be a "nice find." If this change had occurred, it would have still been available in the Creative mode menu. However, Jeb eventually decided against it and added emerald ore back into natural generation after more community response.

As of Snapshot 12w22a, Jeb decided to keep emeralds spawning as he thought it would be a "nice surprise"; Emerald ore now only generates in veins of 1. Because of this, it is much more practical to obtain emeralds through trading.

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.