Emerald

The currency item is a red coin-shaped item used for trading with Villagers.

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.