Emeralds are used to trade with villagers and to power beacons. They are also the rarest mineral in Minecraft and need to be mined with an iron pickaxe or better. Your chances while mining with a pickaxe enchanted with fortune increases the drop by 1 for each level of enchantment up to a max of 4 emeralds. They can be found in Mountain (Extreme Hills) biomes. It can be distinguished by the green "spots" on the stone face.