Block of Emerald

Blocks of Emerald (also known as Emerald Blocks) are mineral blocks that do not naturally generate in maps. Although their primary usage is decoration, they can also be used to store Emeralds compactly. Crafting a 3x3 row of emeralds in a crafting table crafts a single Emerald block. Emeralds can be reacquired by placing the Emerald blocks into a crafting grid.

History
On May 28, 2012, Jeb tweeted an image featuring a room containing Redstone Lamps and Blocks of Emerald.

Trivia

 * Jeb removed the Locked Chest textures in terrain.png to make room for the Emerald Block next to the other icons of items, causing it to have the blank, purple "texture" and the top texture of emerald block.