Although I received excellent rendering training in 2006 at GIA – New York in the Quick Design Lab course, the class dealt only with rendering on vellum with colored pencils and white gouache paint. I love the way jewelry pops when it is rendered on black paper, so after taking the course I did some research and stumbled upon jewelry designer Susan Sarantos’ excellent website. For those of you looking for helpful technical tips for rendering on black paper, Ms. Sarantos provides a step-by-step guide. In addition, the lovely jewelry design books by Maurice Galli, Dominique Riviere and Fanfan Li (please see my post Browse My Jewelry Library for links to more information) give very good instructions especially for rendering diamonds on black paper.
With rendering gemstones in particular, being able to apply very thin washes of watercolor or gouache as a base on the black paper allows you to create the perception of depth in the gemstone immediately without your having to apply darker highlights. You maintain transparency this way as well. When you add white gouache, the dazzling contrast really makes the gemstone come to life. The whole process, when done right, feels effortless!