The Emerald is used to craft:
-Emerald Pickaxe (5 Emeralds, 60 Time Crystals)
-Emerald Chandelier (5 Glass , 10 Emeralds)
The Emerald is a rare gem and you need some time to find it in a cave.They appear to be floating when first seen and can be grabbed as you would pick up an item. It will then make a loud chime noise higher than a Sapphire but lower than a Ruby (you get the point). Emeralds can also be obtained using a Gold Pickaxe/ Shovel on stone and ores.