Speaking of links, as soon as I read this I thought of adamites x-ray mats. They're neon, sort of translucent, good for a see thru fish. We used to have them hosted in the backroom. No direct link anymore but they are still there:

http://backroom.renderosity.com/~bryce/newsite/challenge/shared%20pages/textures_mats.htm
 

I also found the challenge mats at:
http://backroom.renderosity.com/~bryce/newsite/challenge/challenge_resources/pages/challenge_mats_1.htm