Emerald Ore

Emerald ore is a rare mineral block that drops emeralds when mined with a diamond or iron pickaxe. It drops 1 Emerald from each ore block mined (1-4 if Fortune III is used). 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. 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. It is also the only ore with a texture that looks to be small square gems embedded inside the ore, unlike the small, broken rectangles seen on the other ores.

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 32, 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 2 to 0 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.

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 and that one of the snapshots had ruby ore texture instead of the correct texture(emerald).
 * Lapis lazuli and emeralds are the only ore blocks with unique textures.
 * Emerald is the only biome-specific ore.
 * In Minecraft 1.3, mining emerald ore, along with other ores, gives you experience orbs, allowing for a different way to get enchantments. One stack of emerald ore will give roughly 18 lvl.