When a pool of molten bismuth solidifies it tends to form crystals like [the one above]. What seems to give these crystals their characteristic "hoppered" shapes is that there is more rapid growth at the edges of each face than at the center... Hoppering has not been much studied for scientific purposes, but has been noticed in many substances, including galena, rose quartz, gold, calcite, salt and ice.
06 August 2011
A "hopper crystal" of bismuth
Reddit thread the proper designation of the specimen as a "hopper crystal." Here's the explanation re the formation, from Wolframscience -