An Uncut lapis lazuli is a gem obtained from mining Copper rocks or Tin rocks with a Dwarven army axe. Your chatbox will display a message saying that you recover a lapis lazuli gemstone from the ore, along with some extra experience: 17 mining experience. It can be cut with the Dwarven army axe or a chisel into a Lapis lazuli gem, which can be used to make a Lapis lazuli brooch. Crafting the stone grants you 20 crafting experience. When it is cut with a Crafting catalyst equipped, you do not receive a Gem focus. It is based on the relatively rare real gem of the same name, notable for its use in ancient jewellery and the ultramarine pigment it produces.
You have to be mining with the Dwarven Army axe to receive an uncut lapis lazuli. You cannot be mining with a higher metal pickaxe, otherwise you will not receive an Uncut lapis lazuli.