It was kinda tedious, but my formula was this:
Most facets were gradients, going p3 troll skin to a darker mix of that color. Darker at top, lighter at bottom, usually (I lost track a couple times... but it's hard to spot ). Like with gems, how those are lighter at the bottom, for each facet.
Facets which were more-or-less pointed skyward were "lit up" instead with a very light mix of troll skin and white. Viewed from the top, it's almost all "lit up" facets.
Then, I made a very dark blue-grey mix of troll skin and black, and lined the deepest recesses (joints, armpits, the six-pack). This was not precise. More like a wash...
Finally, I edged the tops and sides of facets with a thin white line. The white lines along edges helped hide sloppier bits and add crisp shinyness to what would otherwise have been messy or not very distinctive facets. And that's all there was to it!
I used a lot of acrylic medium for the blending, and learned to never forget: glaze light on dark, and not vice versa...