Emerald

The Emerald is an item used for trading with Villagers. The item can be acquired either by selling objects to villagers or by mining them from Emerald Ore.

History
On May 21, 2012, Jeb released an screenshot of himself testing the trading system. The image shows a GUI with buying and selling areas. The bottom area allows the player to sell various items for currency, whereas the top area uses this currency to purchase items. A possible way to buy them would be to put these currencies in the left slot, where the chicken is, and you can scroll through items for the right slot to take the item you choose once you click on it. The player may use the arrow buttons to scroll through a Villager's inventory and select the desired item. Jeb also mentioned that this may be an end to non-renewable resources, supposedly as most can be bought for some price from NPCs. In the image, a red currency item can be seen.

Uses
As of Snapshot 12w21a, the emerald can only be used to purchase items from villagers. You can sell items to get emeralds from some villagers. It can be mined from Emerald Ore. Once mined, it can be placed into the bottom left slot of the Trading GUI, which allows the player to purchase and remove a quantity of the item the villager is selling. Due to a bug the emerald is called a Diamond if you look at the tooltip.