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.

Usage
As of Snapshot 12w21a, the only use for Emeralds is in exchanging them for items and vice versa. It can be mined from extremely rare Emerald Ore. Emerald ore can only be found in the 'Extreme Hills' biome and the 'Extreme Hills Edge' biomes. To trade Emeralds with Villagers, the player must place them into the Trading Menu's left slot in the amount requested above the slot. The player can then take the resulting item from the right slot. This applies conversely to obtaining Emeralds in trade. They can not be made into tools, armor, or Emerald Blocks.

History
On May 21, 2012, Jeb released an screenshot of himself testing the trading system. At this time, what would become Emeralds were Rubies.

In Snapshot 12w21a, the item was changed to an Emerald. The texture for Emerald Ore remained unchanged, as Jeb forgot to commit the new texture in the Snapshot. The previous Ruby texture remains in the items.png file above the Emerald texture. As this item was not announced before the Snapshot was released, most translations call Emerald "Diamond."