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.