Phosphorescence of calcite and fluorite, exposed for a few minutes to 2537° ultraviolet light from a lowpressure cold quartz mercury vapor lamp, persisted for 18 years. Emission was detected by placing the phosphorescing minerals on black-and-white and Kodachrome film, to obtain “phosphorographs” of the glowing areas. The rate of emission depends on temperature; the color of the phosphorescence has not changed; these confirm the results of other tests that prove that radioactivity is not involved. The patterns of growth of a crystal, and changes of its environment during formation, are clearly shown by the variegated colors in its phosphorograph.
HENRY E. MILLSON and HENRY E. MILLSON, JR., "Duration of Phosphorescence. II," J. Opt. Soc. Am. 54, 638-638 (1964)