Hi Boni, sorry didn't have the chance to get back to this. 

With the basic shape produced by the previous operation (in the image I posted before the red ring). You can select that mesh and edit it more, so just enter object edit mode and grab and move some of the vertices around to get the shape closer. To flatten the top of the signet I would select top view, grab vertices and adjust until they'r flat. To get the chamfered edges, try a sudbdivide or I think there is a chamfer pyrthon script around...

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.