When rocks cool, crystals start to grow from multiple seeds because of thermal/chemical impurities. If the material is sufficiently homogenous in composition and temperature, then the crystals from different seeds will grow at the same rate. When they grow into each other, a boundary between different crystal domains will form. Crystal domains formed this way look like Voronoi cells.

The peanut butter cup looks like this because it's injected from several outlets with an approximately constant flow rate. Each injector outlet forms a Voronoi cell of peanut butter under it.