1. Gems are very simple but quite effective when used as small objects in interfaces, or as components in larger images.
Start off simply with a blank canvas, any size will do. Create an ellipse on a new layer (ctrl/cmd + shift + N) using the ellipse shape tool and use whatever colour you want.
2. Time to break out in the airbrush again. Select your brush tool using settings similar to next for a nice soft airbrush.
Create a new clipping layer (create a new layer by pressing ctrl/cmd + shift + N, then hold alt and move the mouse cursor over the line separating the two layers in the layer box, as shown.)
and put black on the ellipse at the bottom, and white at the top as shown in the image. Don't worry too much about it being very messy, as in the example. Obviously application of this step depends quite a lot on personal taste.
3. The gaussian blur tool will make your airbrushing far neater and smoother to achieve the desired effect. Blur the clipping layer that you applied the black and white to in the previous step by going Filter || Blur || Gaussian Blur, which should make it look something like the example. You may need to use Photoshop's history feature to step back to redo steps 3 and 4 - do this until the image looks as you want it to.
4. Only a little bit left to do; put a small blob of pure white near the top of the gem as shown, blur it with the blur tool set to a very low strength, and tweak the opacity so it doesnt look quite so solid. Lens flares are ideal for gems as a final touch, but dont overdo them. This one has had a background and very low opacity 1px stroke added to it.
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