Emerald is dropped from Emerald Ore. It is the currency used between player and trading with villagers. They are usually found in Desert temples, jungle temples, and very large hill. To trade emeralds or other items for emeralds the player must right click a villager and put the necessary amount of items in the trade box. If this number is correct then the player can pick up the item in the other box. It is much easier to obtain emeralds through trading than finding the emerald ore to mine.
1 Emerald Ore