There's a product in the Citadel paint line called 'Ard Coat. It's basically a clear gloss varnish that you can paint on to a surface that makes it look like crystal or glass. I use it for the Gems on Eldar models and Tyranid mouths/wet-icky parts. It works great, and is easy to use. You might check it out. Any other gloss varnish should work too.