Sorry; I forgot about this. Dispersion for diamond is .044 and I don't understand the multiply node in the shader above, but the idea is basically correct. Calculate the dispersion through the stone and enclose it in a glossy or glass. You can use refract nodes for R,G,and B instead of glass and get the same thing. That is the way it was done by BB in the old firefly threads that are now gone it appears.

The dispersion in a crystal is the change in IOR between the red and blue ends of the spectrum when white light passes through it. All crystal has it but it is only diamonds where we really notice it and that is because diamonds are specifically cut so that the dispersion becomes total internal reflection at the base of the stone. It is the separately refracted coloured light rays reflecting around inside the stone that provides the coloured sparkles. Make your shader with refract nodes and you will will have a firefly shader that provides "fire" in your stone as well.