As lessons online seem non-existant, you might want to get the
Adolfo Mattiello book “Techniques of jewelry Illustration and Color
It is excellent. I would also suggest that you get the templates
that can be ordered separately.
Many years ago I was fortunate to be able to take a workshop in
rendering with George McLean. It was a brief 3 day workshop, but we
covered quite a bit—mostly on rendering different shaped stones. We
worked with neutral colored papers, prismacolors, watercolors,
gouache and pencils of different hardness.
I recently purchased the Mattiello book,and the templates as it goes
into much greater depth, for example, showing rings in different
perspectives. I have decided to spend at least an hour or two a day
learning each of the lessons.
It does not require an expensive outlay for materials. Although
Mattiello states that a drawing board is essential, I have been able
to get along quite well without one, as well as some of the other
items which are listed as essential. One can always improvise.