A bismuth hopper crystal is a crystal with a staircase-like shape that forms when bismuth atoms attach to the edges of a rapidly growing crystal. This process is called "hoppering", and it's caused by electrical attraction that's stronger along the edges of the crystal. The result is that the edges of the crystal grow faster than the center, creating a hopper-like shape (a hopper being a container that tapers downward.) | Rare Earth Gallery