Emerald

Emeralds are a uncommon material in Minecraft and the currency used in Villager Trading. They aren't as rare as you think.

Mining
Emerald ore mined using an iron or diamond pickaxe will drop one emerald. If the pickaxe is enchanted with fortune, it may drop an extra emerald per level of fortune, up to a maximum of 4 with Fortune III. If the ore is mined using a pickaxe enchanted with silk touch, it will drop itself.

Natural generation
In Pocket Edition they can also be found in village blacksmith chests.

Trading
Emeralds are primarily used as currency for trading with villagers. Trading is a much easier and less time consuming way to obtain emeralds than mining is.