A Japanese certified Zentangle artist and teacher Kae Yoshino, has few pretty simple and great tutorials on how to draw gems on her blog and here they are.
First of, she suggests to use watercolor pencils to create a base colors.
Then add more depth to the colors by coloring it more, but with regular pencils.
And now, for the last touch you ad shadowing on the edges, highlight your gems and voila!
Another variation of coloring suggests to draw a dark base before coloring, to create more dimensional view.
Color the base until desirable shades are acquired and then shade the middle with black pencil again, to create more depth to the gem
And for the last touch highlight the gems.
Some of Kae Yoshino tutorials are in English ,but unfortunately most of her blog is only in Japanese language. If you wanna see more of her works feel free to visit the board with her works I collected on Pinterest.
Here are the original tutorials links:
Thanks to Kae Yoshino blog I also learned what is "Zentangle". It is a relaxation form of drawing structured patterns that suppose to carry out wide variety of therapeutic tasks, suck as stress reduction, relaxation, concentration and so on. Was found by Rick Roberts and Mary Thomas and if you want to know more, just visit their official website.