Jade is one of the three fresh additions of mineable resources added by the Erebus. It can be found practically anywhere in the Erebus, although, as with most ores in the Erebus, can only be found between Y=5 and Y=112. You can also get one piece of Jade by combining 9 Jadeberries in a Crafting Table.
It is used for a variety of items.
It is worth noting that Jade has a higher enchantability than Diamond equipment, and possibly even Gold equipment. This means you're more likely to get more enchantments on one item of higher levels.
Jade is required to create Bee Pendants and Beetle Amulets, essential items for using insects to farm. In addition, it is used to create Jade Armour and Tools. Furthermore, if you combine 9 pieces of Jade in a Crafting Table, it'll compress into a Jade Block. Such a process is reversible.
Jade tools are the best tools that can currently be crafted in the Erebus. They have a durability of 863, putting them between Iron and Diamond in quality, with a higher speed than both; however, both the Jade Pickaxe and Jade Paxel are incapable of mining Obsidian.
The Paxel is unique, in that it is an appropriate tool for shovelling, mining and chopping wood; in essence, it is the combination of a Pickaxe, Spade and Axe. To acquire it, for those not using NEI, place two sticks in the usual position and fill the rest with Jade. The Paxel has a durability of 1079, around 66% of the durability of Diamond tools.
The sword does the same damage as an Iron Sword.
Jade armour is also of a quality between Iron Armour and Diamond Armour. It has slightly less than 3/4 of the durability of Diamond Armour, but is more enchantable.
The boots provide 2 armour points (1 armour icon), the leggings provide 5 armour points (2.5 armour icons), the chestplate provides 7 armour points (3.5 armour icons) and the helmet provides 3 armour points (1.5 armour icons), for a grand total of 17 armour points (8.5 armour icons).