1. Paint the gem chaos black.
2. Layer a bit over three quarters of the gem scorched brown.
3. Layer a bit less than three quarters of the gem blood red.
4. Layer about half the gem with blazing orange.
5. Glaze the entire gem with thinned blood red.
6. Paint a skull white dot for reflection at the point closest to your light source.
|Unfortunately this doesn't show the gem at the end of the process, only about halfway through. The final picture stuffed up for some reason but in reality it looks very similar with the layers just smoother.|
The key I find to succeeding with gems is to use multiple thin layers so that the colours blend easily, in fact it doesn't matter too much if they end up being slightly patchy as they will usually be either hidden by another layer or add to the effect. Also here is a list of colour conversions if you're using GW's paint range:
chaos black = abaddon black
scorched brown = rhinox hide
blood red = evil suns scarlet
blazing orange = troll slayer orange
skull white = white scar