Emerald Ore

Emerald ore is an extremely 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. It does not generate in veins as most ores do. Instead it is often encountered as a single block, unlike most ores, and tends to be very scattered around. 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.

Rarity
Emerald ore is very rare, and is in fact 1/40 as common as diamond. That means Emerald ore is currently the rarest ore in minecraft, as only 3-8 blocks of stone in a chunk may be replaced by emerald ore. That means there may be only about 140 emerald ores per every 2000 chunks, compared to the 600 diamond ores found for every 2000 chunks.

Ore distribution
Emerald ore is only generated in Extreme Hills (Edge) biomes. Due to the nature of the generation algorithm, it may be found a short distance into adjacent biomes, but always within the same chunk as an Extreme Hills (Edge) biome. 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.

Smelting
Note: Emerald ore blocks can only be mined with a pickaxe enchanted with Silk Touch.

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, emeralds and Nether quartz 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.
 * The ore itself is considered one of the hardest blocks in the game to obtain, besides the Dragon Egg.