Quote - I have a two-piece setting and diamond earring. They are in two pieces because my experience (perhaps erroneous) has been that if I combine metal object (that needs rounding by Poser) and diamond  object (that I definitely don't want rounded) I will get rounded surfaces on my diamond which looks like poo.

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Or is there another solution I'm not considering?

You can assign different Smoothing ID's to the metal and diamond polygons using Poser's grouping tool.
Selecting the polys shouldn't be much of a problem as you can select by material zone.

This should work for Poser 6 and up...

I've done this only once some time ago, but it's covered in the Poser Reference Manual.

Martin

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