Thanks Don, Sergio!
it is an illusion created with paint, commonly used on miniatures to fake lenses and jewels.
I am no master at it, but it doesn't take too much to create a pleasing effect: Basically, the lens is covered with black. Then a light source location is chosen; I chose directly overhead. A gradient of the lens color is applied on the hemisphere opposite the light source, and a little dot of white is applied on the hemisphere facing the light source.
This fakes the effect of light (a) reflecting from the shiny surface of the lens, and simultaneously (b) refracting through the colored glass of the lens.
Many painters (myself included) will then add a coat of clear varnish on top of the lens to add a slickness to the surface, as many paints can look quite matte.
Like I said though, I am still learning this technique. So I am quite happy I managed to get you to ask this question
