The Emerald is a green-colored gemstone and the third rarest in the game. A fast way to find Emeralds is to use the Spelunker Potion, which makes underground ores and gems luminous. You can also obtain these from the extractinator.
As with all of the gems, selling them is a good way to make some money.
The game will sometimes crash when you ask the Guide for crafting recipes using this item.