It is a bit too small right now, 17mm instead of maybe 25mm. It is small, but almost real scale, as this is small in real world too. I know, when it's too small or too big, strange things can happen during rendering. But I thought, this should still be in range.
I get the same result when I scale it up 10 times... That's not the issue, I think...
Another test: 50mm sphere, same material. Left sphere is not converted, right sphere is converted to a polygon object using c.
The left sphere (not converted) gives me the expected result. The second sphere which is converted gives me this shading thing again.