What is this pattern in the samples' surface called and how/why is it formed? I remember having seen a similar pattern in galvanized sheets.
As often in geology of any scale, the surface is only a manifestation of a 3-dimensional body.
It's typical for igneous rocks (e.g. granite see here) that the polished surface have the Voronoi-like-pattern. The minerals grow from a nucleus until they reach the next crystal and the melt is finished. Every point in the rock can be related back to its closest crystal nucleus, depending on the solidus conditions for the chemical composition.
Check this short animation.
You can see the same pattern in the microstructure of metals, e.g. ferrite and pearlite in steel e.g. here.