Bismuth Hopper Crystals

In the formation of hopper crystals, the outer edges of the crystal grows faster than the interior edge, leading to these angular crystals with a stepped structure. Gaps form in the middle as the inner crystals grow slower and don't have time to fill up that region, forming the "hopper cart" shape. This characteristic is known to occur in a number of other minerals and elements such as calcite, halite, gold, and even in water (snowflakes).

The iridescence is caused by thick/thin layers of bismuth oxide causing light to reflect differently. The bottom image is showing a Non-Iridescent Bismuth Cube next to Bismuth Hopper crystal.