Now that you have your blacked out gem, pick a corner to lighten towards. I find that eyes tend to look better if they're lightened towards the bridge of the nose (centre-top of helmet), and that jemstones work best if a diagonal is taken, but you can experiment yourself and find what works for you. In both examples, I will use the top left corner as my light point.
Taking a dark shade of the colour your gem will be, fill in the stones shape, leaving one corner black, and also leaving the outline black - take your time here, this is the most time consuming part.
For reference, the colours used here are : Left - Dark Angel Green. Right - Red Gore.