In this post I'm going to describe how I got a jade effect on my mask. Searching Google trying to figure out the best way of doing this tells me there are a few people out there who want to know how to paint faux jade, so I'm going to share my method.
I started out painting the whole thing white. Not just any white: bright titanium white. The white base coat can be a little patchy, but not too patchy. I've then got viridian, ultramarine, and yellow ochre. I've put some splashes of ultramarine and yellow ochre on the white, and splashed a watered down mix of white and viridian on top, with some darker viridian areas. I've used a brush with stiff bristles in a stippling motion, which helps the paint to layer and blend nicely. It also makes sure you get a nice stone texture with no tell-tale brush strokes.
These pictures show a close up of the blotchy blue and yellow undercoat, and a close up of the final effect before I glazed it with structure gel (I'll get to that in a minute).
I finished up by smoothing structure gel all over the surface. The gel dries smooth and translucent, which gives some depth to the stone effect. The secret to getting it on nicely is long, gentle strokes with your finger and not too thick of a gel layer. You can always do another coat.
A wash of very dark red-brown is also helpful for showing up the mosaic effect I put on the skull, and adding some depth to the "jade". This wash really improves the realism of the effect, but if you're doing this you'll need to make sure it's very, very thin.