Hello,
I think the matter: gems, have a chaotic behavior.
sometimes have to force transparency, sometimes not.
http://www.screencast.com/t/ALa7l084olZ
Thank you check.
Antoine
Increase your ray bounces, enable global & ground illumination, and add a ground plane (Edit > Add Geometry > Ground plane).
Gem stones are cut to specific shapes for a reason, a simple sphere will not give you the refraction and sparkly appearance that a cut stone will.