For the template that I use, I dup then do color to alpha (white to transparent) on a duplicate layer. I then dup this layer and set the top layer to Grain Extract and merge down. I then run Rod's script-fu to fill in the trasparent regions. I set this layer aside for now.
For the untouched base layer, I dup, then color to alpha again and dup that layer setting it to Grain Extract and merge down to make a gray outline. Move this outlined gray layer to the the base, I then run G'MIC's Polygonize per the flow on both these layers setting to difference (don't forget to add alpha channel to the non-touched original). You should now have a crystalline
outline. Do auto-equalize and enhance contrast steps to really make the result look crystalline
. Now that you've done this step, you get the layer we originally created and delete out all the areas other than the fill regions that you want to keep; this is your glass fill layer of course. You lower opacity to taste and voila.
Now I embellish a bit doing blends of G'MIC's Local Orientation (increase smoothness slider a bit) set to overlay. You have to play to get the look that you want in the end. Sparkle and Supernova effects at the end.