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 in groups of one, but they are very hard to find so it is much easier to obtain them from villagers.

Uses
Emeralds can be used for trading with villagers, as well as powering the beacon and building a pyramid-socket for it out of blocks of emeralds, but cannot be used to make tools or armor. 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. Trading is a much easier and less time consuming way to obtain emeralds than mining is. Trading with villagers provide the player a variety of items, many which are hard to get and not renewable otherwise. This is why emeralds could be considered important for acquiring other resources.

Smelting
Note that the emerald ore block cannot be obtained without the Silk Touch enchantment. If an emerald ore block is obtained, it is much more efficient to place the block and mine it with a pickaxe with the Fortune enchantment.