Drawing gems is more easy than you would think. It is an easy technique that you can apply anywhere you need to do gems. Be it photoshop, drawing, painting etc. I learned about it roughly 10 years ago, while I were painting Warhammer 40k miniatures, but only just recently applied it to photoshop. In this tutorial, we'll assume you are using Photoshop.

	Draw a sphere in a new layer. Make it roughly 25x25, and fill it with a neutral grey. Tip: to make a completly round circle, hold down shift while using the Eliptical Marquee Tool. Enable Preserve Transparency in that layer.
	Use a small paintbrush and make a few white dots at the bottom of the sphere, and some dark dots at the top, like shown in the illustration. It is important not to draw too close to the edge.
	Make sure the Preserve Transparency is on, and use gaussian blur at a setting of 3 to 4 pixels.
	Dublicate the layer, and set the new copys layer blending to color dodge and the opacity to around 50%.
	Dublicate the copy, and set the the new copys layer blending to color burn. The opacity of this new layer should also be 50%.
	Paint in a small highlight at the top manually, and the gem is now finished. You can color it in any way see fit. If you want to create a bigger gem, you would have to work more on subtle highlights.
	When coloring, I usually use a hue/saturation layer set to colorize, and the hue around 180 to 200 for a nice blue tone.