Emerald

A Emerald is an item dropped when Emerald Ore is mined or more commonly obtained by Trading with Villagers.

Uses
As of Snapshot 12w21a, the only use for Emeralds is purchasing items from Villagers. The player can also sell items for Emeralds. It can be mined from extremely rare Emerald Ore. 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.

Emerald ore does give emeralds, but tools, armor and emerald blocks cannot be crafted like other ores, acting very much like Redstone and Coal.

History
On May 21, 2012, Jeb released an screenshot of himself testing the trading system. At this time, the Emerald was a red coin-like object, hypothesized to have been another gem, such as 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 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."