>
Water of crystallisation is water which is bonded in crystals by chemical reaction.

Left:      red cobalt chloride with crystal water (CoCl2.6H2O)
Photo 2: blue cobalt chloride without crystal water  (CoCl2)

Photo 3: transparent calcium sulfate with crystal water (CaSO4.2 H2O)
Right:      white calcium sulfate (CaSO4).