Emerald

"Ah, to be minted with the green stuff. To swan about the villages of the Overworld with pockets bursting with emeralds. “Ho there, villager!” you would bellow, tossing a few emeralds for a handful of carrots. You’d never need farm again!"

- Alex Wiltshire

Emeralds are an uncommon mineral, and are used in trading or to power beacons.

Trading
Emeralds can be obtained by trading with villagers. Villagers can be found in villages and will either buy or sell specific goods for emeralds.

Mining
Emerald ore is mined using an iron or diamond pickaxe and 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 emeralds with Fortune III. If the ore is mined using a pickaxe enchanted with Silk Touch, it will drop itself instead of an emerald.

Drops
Vindicators and evokers will drop 0–1 emeralds upon death.

Foxes that are holding emeralds also have a chance to drop the emerald they are holding upon death.

Vindicators and Pillager spawn from raids can drop more emerald by

0-1 Emerald (up to 4 with looting III) And has small chance to drop one of these extra emerald:

2-3 Emerald (up to 6 with looting III) 4-5 Emerald (up to 8 with looting III)

Trading
Emeralds are primarily used as currency for trading with villagers. Trading with villagers is typically a faster way to obtain emeralds than by mining, as emerald ore is very rare.

Beacons
Emeralds can be used to select powers from a beacon. The player must select one of the available powers and then insert an emerald into the item slot.

An emerald can be substituted for an iron ingot, a gold ingot, or a diamond.

Trivia

 * The Ruby was going to be the Villager currency, but was changed to Emeralds before the update's release.
 * The Emerald actually matches the Villager's eyes. It is unknown whether or not this is intentional.