It's because it's transparent. Most use corian (white or bone) to carve litho's. Either 1/4" or 1/2" (I've only used 1/4") seems to work and most seem to buy on ebay--do a search and you'll find sellers offering 8x10" sheets. You'll also find posts on here with Corian or equivalent periodically. I'm sure other's will respond with their favorite sources. Below is an example
http://cgi.ebay.com/3-Pcs-1-4-Corian...item439e964b1b
---------------------------------------------------------------
George
Life is short....drink the good wine first!