I made my xbox jewel clear and put on the case. I cut a hole where it used to be and left a little ring around the inside for the jewel to sit on. I was going to just buy a 80mm green cathode fan and put that under the jewel. Would that be the best thing to do though? If not what are some other ways to light up the jewel

CC's are the way to go.

You can use LED's as well for cheaper. Also you can get a more true color, like red cold cathodes are pink sometimes but LED's are always red. If you want to use LED's, get them from ebay or lsdiodes and then use my tutorial in my sig.

someone told me if the jewel was clear it wont look green with a green cold cathode. they said to sand down the back a little to make it hazy.

whoever told u that has no idea what they are talking about