Thank you all for your inputs!
It really is a MeshToNurbs object (I received an stl-file from the scan of the stone. In order to build a bezel setting around th stone I converted the mesh to a nurbs object).
So the “pen” or “artistic” display didn’t change much. And with the MergeAllFaces command - I guess there are to many data - Rhino didn’t come to an end.
If I uncheck “show isocurves” in the shaded view some lines disappear but there are still lots of…
Is there maybe a better way to get started with my stl-file?