BUYER'S GUIDE: MOONSTONE FELDSPAR This is the ninth in a series by Senior Editor Robert Weldon designed to help jewelers buy different types of gems. What to look for: Moonstone has an aura of mystery &endash; and for good reason. Properly oriented, the best moonstones glow with a shimmering blue light against an otherwise colorless, transparent to semitransparent body color. This glow seemingly floats over the surface, appearing and disappearing as you tilt the gem. Gemologists call this glow adularescence; geologists call it the "schiller effect." By either name, it's what makes moonstone special. Adularescence is caused by the intergrowth between two types of feldspars: orthoclase and albite. The two feldspars have different refractive indexes, and that's what produces the unusual, reflective optical effect in certain directions of light. The key to revealing adularescence