Emerald

Emerald Ore
Emerald Ore is an extremely rare block that drops Emeralds when mined with an Iron Pickaxe or better; it does not require smelting. It is only generated in Extreme Hills and 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. The ore spawns anywhere between layers 2 and 31, meaning it can occur as high up as gold can, and is less likely to occur at the bottom 4 layers of bedrock. Veins of it are as large as veins of Lapis Lazuli.

Emerald
An Emerald is an item dropped when Emerald Ore is mined or more commonly obtained through Trading with Villagers. The occurrence of their respective ore is very limited in the world.

As of Snapshot 12w21a their only use is as a form of currency in exchanging items with villagers.

Block of Emerald
Blocks of Emerald are mineral blocks that do not appear in normally generated maps anywhere. Although their primary usage will be decoration, they can also be used to store Emeralds[1].