You get crystals when a saturated solution evaporates. Water (the solvent for salt) can only hold a certain amount of dissolved salt. If you have a saturated solution, then any evaporation means that the remaining water can’t hold the salt contained int he original solution. As the water evaporate, the salt naturally forms into cube-shaped crystals on nucleation sites (irregularities) in the container.

You can experiment with this in your kitchen by dissolving as much salt as you can in warm water. Put the water in a wide-mouth jar or a glass, tie a couple of cotton strings to a chopstick and hang them in the water. As the solution evaporates, crystals will form on the strings. If you put a drop or two of food coloring on the string, the crystals will grow in color. There are also formulas for creating crystal gardens with Epsom salts, or with a combination of salt, blueing and ammonia. We used to make these when I was a kid. Facevamo strutture con stuzzicadenti immersi nel colorante alimentare e facevamo crescere intere città con torri, ponti, case, ecc.

Giardino magico di cristallo fai da te!

Come crescere un giardino di cristallo | Howcast