Magic crystal
Originally usable only after reaching level 51, Magic Crystals allow a character to expand their maximum mana from 200 up to 300 at a rate of 10 max mana per crystal.

They can be crafted anywhere using 2 Mana Crystals, and crafted back into their components anywhere as well.

It is unknown as to whether or not Nakano intends to return the item's use, but as of all N Terraria updates past Terraria's 1.2 update, it has no function.