Motivated by the wonderful "Gemstone Challenge" on the Shading Zentangle Facebook group, I'm writing at the speed of light a new ebook on how to shade and color smooth glassy gemstones. Here's the potential cover:
Follow this link to get the ebook: http://www.enioken.com/3dtangle/gemstones
The challenge was inspired by the glorious work by Kae Yoshino, who posted a couple of tutorials on how she draws her own gems.
You can also check out my Pinterest board "Drawing Gemstones" which has many online free tutorials.