Minecraft wiki entry for Lapis_Lazuli, associated text: In real life, lapis lazuli is a blue gem that can be ground and processed into ultramarine pigment. Lapis lazuli pigment has been famously used in the production of illuminated manuscripts, stained glass, and cave paintings. See the Wikipedia article for more information.
Lapis lazuli is the only ore that can be used as a dye‌[BE only] or making a dye.
It is the only item that can be put in the second slot of an enchantment table.