What causes the stairstep structure in bismuth crystals?
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.