A special optical occurrence, or circumstance observed or are visible in certain gemstones such as chatoyancy, asterism, or play of color, adularescence, girasol, labradorescence, opalescence, schiller, etc.
See Alexandrite Tsarstone collectors guide, Definition of Phenomenon, http://w.alexandrite.com/viewpage.html?id=GG-101-00004 (A special optical occurrence, or circumstance observed or are visible in certain gemstones such as chatoyancy, asterism, or play of color, adularescence, girasol, labradorescence, opalescence, schiller, etc.) (as of May. 31, 2026, 02:21 GMT).