I'm using purple gamma changing the tone a little bit. The brush and pen pressure does everything else.

Now let's make the side reflections.

Use Lasso Tool. Make this kind of selection.



Create a layer.



Choose this gradient and hold your breath.







Also apply a mask to hide the part of the effect.



Moreover play with opacity settings.

Create 2nd edge.



And, so on:











Now maybe the most important, if you a making the gem with such Polygon edges the reflections got to be correct.

I've used this Gradient to hide or expose my effect, my white gradient fill in this case.







Now a little general reflection and we are done here.



I've also desaturated my background to pop up the gem. Well this is it i hope you enjoyed and learned something from my tutorial, and I welcome you into my nest tutorial on making the stones.