Emerald Ore

Emerald ore is the rarest mineral block in the game. It drops emeralds when mined with a diamond or iron pickaxe without the Silk Touch enchantment. It drops 1 Emerald from each ore block mined in this way (1-4 if Fortune III is used) or experience orbs worth 5-7 experience if mined with any other tool (including a pickaxe without Silk Touch.) Instead of generating in veins like other ores, it is encountered as a single block, 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.

Ore distribution
Emerald ore is only generated in Extreme Hills 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 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. In a world there are about 0.2 Emerald ores per chunk. It will only generate in locations which would have otherwise been stone - it is thus possible for as few as 0 to 2 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. Also note that even though emerald ore is less common than diamond ore in general, it is possible for emerald ore to be more common than diamond ore in a certain type of biome or a specific chunk.

As a smelting ingredient
Note: Emerald ore blocks can only be obtained directly with a pickaxe enchanted with Silk Touch. Otherwise they will drop emeralds when mined.

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).
 * Emerald and Nether Quartz are the only biome-specific ores.
 * 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 levels.
 * Emerald ore, along with Lapis Lazuli ore and Nether Quartz ore, are the only ores that have a unique texture.
 * Ruby ore may not be implemented again, as players could confuse it with Redstone Ore
 * Emeralds are used as the core currency within NPC villages. They can also be obtained by trading a specified item for an emerald.
 * Even though emerald ore is rarer than Diamond Ore, emerald ore is found more often in the extreme hills than diamond. This is because it is scattered.
 * Diamond Gems and Ore used to be called Emeralds.