two types of asterism: Epiasterism, such as that seen in sapphire and most other gems, is the result of a reflection of light on parallel arranged inclusions inside the gemstone. Diasterism, such as that seen in rose quartz, is the result of light transmitted through the stone. In order to see this effect, the stone must be illuminated from behind. Rose quartz also exhibits epiasterism. See also Isomorphism (crystallography) References D. S. Phillips, T. E. Mitchell and A. H. Heuer,"Precipitation in Star Sapphire I: Identification of the Precipitates, Phil. Mag. A, 1980, v. 42, N0. 3, pp 385–404 Gemology Optical phenomena