Intersection smooth/polyhedron - how to deal with the normals?

[stla]

Hello,

I've just played with the intersections. This is really amazing. Here is the intersection of a sphere and a tetrahedron:

Good. But here is the rendering after adding the normals:

Not very nice: the "corners" are smoothed. How would you deal with this problem? Is there a way to use the normals for the spherical part only, or something like that?