A line runs through the center of a crystal, to which all faces of a given zone are parallel.
See Alexandrite Tsarstone collectors guide, Definition of Zone axis, http://m.alexandrite.net/viewpage.html?id=GG-085-00003 (A line runs through the center of a crystal, to which all faces of a given zone are parallel.) (as of May. 21, 2026, 17:49 GMT).