Emerald

Emeralds are items used as "currency," normally obtained by, and used for, trading with villagers. They can also be found in desert and jungle temples, and by mining emerald ore, found sporadically in Extreme Hills biomes.

Uses
Emeralds can be used for trading with villagers, as well as powering the beacon and building a pyramid-socket for it out of emerald blocks. They cannot be used to make tools. To trade with villagers, right-click them, and a trade menu will appear. 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.

Bugs

 * Sometimes, when shift-clicking items from trading, the emeralds will glitch and will not be able to be used. The same goes for all items. This is because if the trader decides to not do that certain trade anymore, any leftover items given to him will be dropped.
 * Sometimes, if the game dumps to Windows while in the middle of trading, you will receive the item but all emeralds that you have in your inventory will be gone.