These are some of the spheres I've been rendering while taking Steven Zapata's "Form from Imagination" drawing course. They're laid out in roughly the same order as I made them.

I don't think I'm consistent at these, but I think there's a clear improvement between my first attempt at each (front-lit and back-lit) and each subsequent one.

One thing that I can definitely improve on: the penumbra of each shadow. Other than the rather poor attempt on the third row, I skipped adding it entirely.