How do the crystals form?

Answer 1

Generally crystals form when a fluid substance solidifes or precipitates, if it's a solution. In this process, atoms get progressively close to each other, and at some time they bond at some specific angle with one or more atoms close to itself. This angle will the depend on the atoms and molecules themselves, but they will always bond in a way to decrease their energy and become more stable.

This crystal process can progress in 2D, all crystals forming as layers in more or less parallell planes, or 3D forming increasingly large solids - cubic shape, tetragonal shape, etc.