An Emerald (or Emerald Block) is a shiny bright green block with some cracks on it.
Emerald Blocks be found in Forest Mines, Mountain Mines, Arctic Mines and Cavern Mines. They cannot be mined by breaking it, but rather using a Bomb to blow them up.
An Emerald can be destroyed when using a Bomb near it in mines (Except Cavern Mines). Ruby and Diamond also have this mechanic.