The Patricia Emerald Crystal
The Patricia Emerald Crystal was discovered in the Colombian Andes' Chivor Mine in 1920 and is named after the mine owner's daughter. Not only is it one of the largest gem-quality emeralds in the world, but it is also dihexagonal, or 12-sided, and measures 2½" × 1".
The stone has some flaws, but this is common in emeralds -- the vast majority of faceted emerald gems have flaws. The stone resides in the New York Museum of Natural History.