This is a tough one because i'm sure there are different interpretations of a "Crystal Ball" but if I'm correct in my vision.. i'd first create the sphere... this tutorial will help:
BioRUST.com :: Tutorials >> Shiny Spheres
and then fill it with mist or fog using brushes such as these:
Download: Photoshop Brushes Series 42 (thank Bedlam123 for the site link)
if you want the ball to sit in a wooden base or something similar, i would create a rounded rectangle shape underneath, and then use the EDIT:TRANSFORM:WARP to give it the shape you desire. Then apply a texture such as one of the ones available here:
BioRUST.com :: Downloads >> Adobe Photoshop Layer Styles
(Check woodworld or naturalstone)
Hope this leads you in the right direction. If you end up with a product you really like, why not make a quick tutorial to let others know how you did it!
Mike