Water molecules, which have been chemically combined in certain crystals or hydrated salts, which are given off when the crystals are heated.
See Alexandrite Tsarstone collectors guide, Definition of Water of crystallization, http://w.alexandrite.net/viewpage.html?id=GG-082-00001 (Water molecules, which have been chemically combined in certain crystals or hydrated salts, which are given off when the crystals are heated.) (as of Apr. 09, 2026, 09:45 GMT).