Today i lowered absorptions for my fractured Amethyst model and caustics got much better.
If you are bored and wold like to experiment with the materials (in materials LXM file) to make it even better or just study.
Or you can send me FLM, why not (i use LuxRender 0.8 final).
It renders fast and needs ~ 300 Mb for LuxRender 0.8 master.
I share here the scene just un7zip it and it's ready to go with LuxRender 0.8 final.https://docs.google.com/open?id=0B1ETgeadYhb8N2Y0ZTI2NzctMTA0ZS00OGJlLWIxZDgtZTRkOGFjZTJhYzMw
I have used UV texture for some parts of the gemstone shader might be better.
Render looked good in 400 S/p when i did low res test (800x500 pix), it took me ~ 1 hour with c2d e4500 + c2d e7400 (i5 and i7 are faster than my combo
Now i get 100 S/p in 55 minutes.
Tone Mapping (the HDR) is too blue, need to tune it somehow.
Oo and there is no SSS fog in scene, don't let the filenames to disturb you (there used to be, but it was too slow that way