So how was it done?
It is actually very simple.
But first a couple of restrictions:
#1 this currently (M2.5) only works with raytraced shadows, they allow transmission of light (color).
#2 it only works for transmitted light (so no reflected caustics)
Then to the setup:
-Create a shader, I like blinn (though I never hugged him..)
-Create samplerinfo node
-Create a ramp
-Take the facingratio og the smaplerinfonode, and feed it to the UVcoord->Vcoord of the ramp
-Then feed the ramp outcolor to the transparency.
This makes the transparency of nonfacing parts correspong to bottom
of the ramp, and the facing pats to the top of the ramp.
The trick is to make the transparency of the facing part more than 1 (1,1,1 for rgb) this intensifies the transmitted light/color, faking the focusing of light.
In effect I make a very narrow part of the top ramp have high intensity (ultra transparent), a very narrow part of the bottom vaery low intersity (opaque), and the large middle area I give a netrual intensity (in fact I have made tests with a cloud texture for the middle part to break it up a bit, see fake_caustics3.jpg).
To create the colored transmission, I have colored the Colorgain of the ramp in the same color as the gemstone.
Basically all the interesting stuff happens in the ramp!