Block of Emerald

Blocks of Emerald are mineral blocks that do not appear in normally generated maps anywhere. Although their primary usage will be decoration, they can also be used to store Emeralds, just like the other Mineral Blocks. This may be thought as the hardest item to acquire (with the exception of blocks requiring "silk touch"), taking the Block of Diamond's place, because of the Emerald Ore's specific-biome generation.

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