Alunite

Minerals are composed of atoms, ions, or molecules.

In crystallization, these atoms, ions, or molecules are ordered so that they become crystals (geometric shapes that we can see at a glance)

Alums are easy to crystallize. Alums consist of one molecule of potassium alum and the other of a metal sulphate (usually potassium or copper sulphate, although it may also be iron).
The crystals form under the earth's surface. Water, temperature and pressure are the environmental factors that allow the formation of crystals, and shape their structure. Each crystal has a different chemical composition, on which depends the structure and size of the crystal.

The obtaining in this case of the alunite the method of evaporation. It consists of supersaturating distilled water with potassium alum. The filtrate is optional but allows to remove impurities. We allow the solution to evaporate slowly. The slower the process, the crystals will be larger.
The crystals of alum crystallize octahedron form. To make them bigger, the solution must be renewed. The food coloring makes them more colorful.

Bibliography and more information:
http://cristalizaciondeminerales.blogspot.