Hopper crystals

When a pool of molten bismuth solidifies it tends to form crystals like those in the first two pictures below. 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. (Spirals are probably associated with underlying screw dislocations.) 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.