I needed a scheme for painting gems on my Eldar. The easiest way to try it is on larger shapes that have a similar surface shape, and I had a couple of wraithguard heads sitting around. I was amused that the result ended up looking like anime eyes at this scale. Heh.
For the green version it was a basecoat of black, Pthalo Green base, bottom highlight was Pthalo Green/Slimer Green, Slimer Green, Slimer Green/Warm Yellow, white edge then Warm Yellow glaze, top highlight was Pale Blue,
For the blue it was Red/Pthalo Blue to make a purple, the Pthalo Blue and white for the brighter colours, and a little of the Pale Blue in the final highlights.
This approach gave me a useful idea quickly of how I wanted the colours to work against red and white.
Footnote for the Eldar purists. No, I know these aren't gemstones they're instrument clusters because they don't have a gemstone setting ring around them.