My personal favorite is milk glass. Wikepedia describes milk glass as an opaque, milky white or colored glass, that is blown or pressed into a wide variety of shapes. It is also used in dinnerware, lamps and vases. When used in costume jewelry, it produces a piece with a crispness not otherwise found.
Early examples of milk glass vintage jewelry include pieces marked Germany, and Japan. Miriam Haskell was particularly fond of milk glass in her jewelry designs, and her pieces are much in demand and command high prices. Trifari also used white milk glass, particularly in marquis and navette shaped stones. This gorgeous milk glass flowered brooch is a new addition to the Trifai page of my Vintage Jewelry Lane Website. It is so simple in design, but just elegant.
Milk glass is used in all sorts of vintage jewelry. Japan and Germany made both earrings and necklaces - some single stand, but many with multistrands and fancy milk glass clasps.
Germany and Miriam Haskell often used unique shapes of milk glass instead of the traditional oval or round shaped often used. Dimpled glass or "baroque" glass shapes like the one shown here were common. Some designers used petal shaped milk glass stones in earrings for a lovely flower petal effect.
Not all milk glass vintage jewelry pieces were marked. Some of the nicest designs that I have seen use prong set milk glass stones in quality settings, but have no markings at all. Often, these are unsigned designer pieces. Designers often had pieces made for their high end lines with their signatures, and then had some other pieces for the lower price ranges with no markings.