Emerald Ore

Emerald ore is the rarest mineral block in the game, which drops emerald when mined.

Obtaining
The emerald ore block itself (rather than its emerald drops) can be obtained by mining it with an iron or diamond pickaxe with the Silk Touch enchantment.

Natural generation
Emerald ore is only generated in extreme hills biomes (which actually contain more emerald ore veins than diamond ore veins). It'll even continue to stay that way when it's customized. 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, for example in river biomes which cross through them. 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 that 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.

Usage
Emerald ore drops 1 emerald and 3-7 experience when mined with an iron pickaxe or better. Mining with a Fortune enchanted pickaxe will increase possible yields by 1 per level ( 1-4 emeralds per ore with level III). If mined with any other tool, it drops 5-7 experience.

Trivia

 * Even though emerald ore is rarer than diamond ore, emerald ore is generally found more often in extreme hills biomes than diamond ore, as the emerald ore blocks are generated in a more scattered fashion.
 * It is possible to have two or more emerald ore blocks generate adjacent to each other, though this is very rare.
 * As emeralds are far easier to obtain by trading with villagers, the player may wish to use the silk touch enchantment to collect the far harder to get ore.