I can posit a theory:<p>A hexagon is a boundary shape with the least energy. What you are really probably seeing is a circle that has pushed outwards against another set of forces and reached equilibrium (differing temperatures, gasses, who knows what). I have been playing with voronoi diagrams and diffusion limited aggregation recently , and it is amazing some of the structures that will emerge when a system reaches equilibrium. It is possible that certain forces have aligned with their peak energies in the form of a regular polyhedra, which happens all over nature on large and small scales.