[view] • Lapis Lazuli is an ore drop that is used for decoration and enchanting that can also be acquired by trading with clerics, and the rarity of the Ore is balanced by the drop quantity of 4-8. For decorative purposes, Lapis Lazuli can be used to craft Lapis Lazuli Blocks, used as a blue Dye, or mixed with a Rose Red, Bonemeal, or Cactus Green to make purple, light blue, or cyan dye.
In real life, lapis lazuli is a semi-precious-stone used for making jewelry or as a pigment.
Lapis lazuli dye was going to be a crafting ingredient for the block crying obsidian, but neither the block nor the recipe were ever added.
In 1.2.5, when the pick block key was broken, if you used it on lapis lazuli ore, you would get an Ink Sac, for Lapis Lazuli and Ink Sacs both have the ID of dye, and Data Value 0 is Ink Sac. This along with many other glitches were fixed in the 1.3 release.