Gemstones, while being fairly small physically on a miniature, have an amazing impact on the overall look of the model / unit / squad when a little time is taken to do them justice. They are often overlooked, so I've chosen them for the first of my painting guides for the site. Also, many sci-fi themed miniatures have enclosed helmets or visors, so as the method of painting is the same, I'll use a Warjack / Space Marine's eye as the second example (above right).
Thankfully, these are pretty quick and simple.
Firstly, basecoat the area black, and leave to dry. This saves having to painstakingly paint a black outline in later.