Silica, quite basically, is sand. I think what they are talking about is raw glass or quartz glass, which is what they used to use for windows before modifying the glass to block the UV rays that were responsible for curtains and furniture losing all their color. Old houses with their original windows have that property -- good for our herps, bad for our furnishings. LOL
Never heard of plexiglass allowing UVB transmission -- if that's true, that would be great! I've always read that you have to remove the plastic cover off of any light fixture you get because it blocks UV.
Thanks for the links, they are interesting! Looking forward to other people's input on this. 
Julie, Justin and Frasier the Veiled.