In the hand -painted gem jewelry renderings. Is it enough to draw a round shape with a ruler? Do you still use watercolor or gouache? Good performance effect?

If it is expressed in color, it must be better. But for gem design, the important thing is not the renderings, but the cut surface of the plane map determines its luminescence. The more the cut surface of the gemstone, the stronger the refraction light, and the stronger the luminescence

Watercolor and gouache are easy to change. It is still the way to draw watercolor with watercolor.
The book to draw this book according to the steps, practicing proficient in practice, and studying other people's paintings by themselves. Bulgari and Cartier have many perfect design drawings to learn from and learn.