Lots of fantasy models include gems on them, wizard staves, sword hilts, fancy shields, etc... and painting them to look good is pretty easy. They can really help give a model a 'finished' look.
I have broken down gem painting into two methods, the 3 step, and the 5 step. I generally use the 3 step for small gems and the 5 step for larger ones. Enjoy.
Step 1: This is a red gem, so I basecoated it in a dark purple. I like to use a cool color for the first layer.
Step 2: I painted roughly 50% of the gem red. Try to leave the dark area in a round shape. I like to keep the dark area near the top of the gem. Not sure how realistic it is, but it looks sharp.
Step 3: Add a white dot near the top of the gem. Finished! Wherever you put the white dot, make sure it is touching the dark area for maximum contrast.
Step 1: Now on to the 5 step gem. I start with a cool dark green for the basecoat.
Step 2: Paint 60% of the gem with a lighter green.
Step 3: Inside that green, I layer an even brighter green.
Step 4: I mixed some white into my last color for a final layer.
Step 5: Add a white dot to the dark area. Done!
After you spray your model with matte varnish, go back and paint the gems with a gloss coat to make them shiny again.