There are three kinds of glass known as opalescent. One is blue-tinged, semi-opaque or clear glass with milky opalescence in the center. The colour is produced by the slow cooling of the molten glass in those parts that are thick causing some crystallization inside the glass. This contemporary opalescent glass was first produced in the 1920s and 30s by companies in France such as Lalique, Sabino and Jobling.(Source)
posted by Admiral Haddock at 7:50 AM on September 9, 2011