If you rig it as a figure, the diamond and the ring will be two separate body parts,
and you can set the diamond for No Polygon Smoothing and the
ring for Yes Polygon Smoothing. 

i.e., the paragraph for the diamond would end with something like

    displacementBounds 0
    shadingRate 0.2
    smoothPolys 0
    }

and the paragraph for the ring would end with

    displacementBounds 0
    shadingRate 0.2
    smoothPolys 1
    }

Or a simpler solution: Use UVmapper or your modelling app to split (or bevel)
all the vertices of the diamond.  Poser will then leave it sharp and flat.

My python page
My ShareCG freebies