Halo Effect. A toning on a coin or medal around its circumference darker than that in the center surface. Silver coins are susceptible to such toning particularly when placed in albums or cardboard holders which have been manufactured with sulfur in their processing. The close and constant proximity of the silver to the sulfur paper will, in time, cause the toning, more so near the edge than the center.
excerpted with permission from
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor