A term used in crystallography to the direction of an anisotropic crystal along which there are no double reflection.
See Alexandrite Tsarstone collectors guide, Definition of Optical axis, http://m.alexandrite.net/viewpage.html?id=GG-100-00002 (A term used in crystallography to the direction of an anisotropic crystal along which there are no double reflection.) (as of Apr. 03, 2026, 15:36 GMT).