What causes the stairstep structure in bismuth crystals? I've been interested in bismuth crystallization since I found out they existed, but have a question on their formation.

According to Wikipedia, the stair-step structure of bismuth crystals is "the result of a higher growth rate around the outside edges than on the inside edges."

What would cause this higher growth rate around the edges? Cooling?
Hopper crystals, that is, the "staircase" form, develop when a crystal forms so rapidly that material is used up before the interior finishes.

Table salt, $\ce{NaCl}$, also forms nice hopper crystals. If you want to try this, use "kosher" salt or CP $\ce{NaCl}$, which doesn't have anti-caking and iodide additives that would spoil the crystal structure.