Liz answered this one at the beginning of the post. Ray-Ban
G-15 lenses are made from glass. Lenses made from glass are usually heavier than the lenses made out of plastic. Ray-Ban
uses a special technology that make glass lenses light weight without diminishing their optical
quality. To indicate that their lenses are lighter that most glass lenses in the industry, Ray Ban
calls their lenses "XLT", which stands for extra light.
G-15 means green. To quote Ray-Ban
exactly they are their trademark neutral green also called the natural vision lens. It offers a 85% light absorption and 100% UV protection.
I hope this helps.