Made this on a sunday evening.
Its simpler than it looks. Not much going on with my usual shadow/glow alchemy, except for a white inner glow from center.
Basically just two layers with fg-bg gradients (orange to purple and green to blue-gray) split by a layermask.
A small inner bevel and the big outer bevel.
That shadow/reflection thing is motionblur and the background brushes.
Oregonian on Gimpchat asked: "what he means by splitting with a layermask ?"
(and btw, when she says he, she means me not Sauron, lol).
Here is a screenshot of the layermask and gradient-layers: