This October birthstone often has a white background buy displays a lot of colors in bright light
.... White or light opal: 
Translucent to semitranslucent, with play-of-color against a white or light... [DOC] [TLE] Gemstone Jewelry History & Information on Gemstones & Gemstone ... [PAR] This gemstone jewelry is often sold artificially to reduce high semi-precious gem 
... of alexandrite that are light sensitive, so be careful when buying the natural ...