Off the top of my head, you might try a radial gradient fill instead of the angular. White in the center to black at the edges. You might also want to apply the gradient in a non-linear manner (or adjust the curve later). You'll also want to -- probably need to, really -- add some noise and blur to break up the sudden drops you'll get at the edges. The gradient generally won't go to zero there, so you'd end up with a bunch of sudden drops.
On that note, if you want the peak to be not quite in the center, apply a small displacement map to shift it. Don't go too big or you'll get what I call 'stretch marks'.
After that, continue as before to get the surface texture and the bump maps.