Gems!

I start with the darkest colour visible in the gem,
this can be really quite dark

Painting on the side opposite from the light source trying to keep this layer nice and oval, this can cover about 80% of the surface.

Starting to push up the highlights, like most shiny materials we want to make sure theres a high contrast between the darkest & brightest colours.
This covers about 30-40%

Finally pushing up the highlight to the brightest value exactly opposite the light source (top right)

A small bright spot to indicate the direction of the light source, and we're done!