For silk, the material is strange in that it has a high contrast within itself and usually quite vivid colours; within a relatively narrow angle it reflects light quite a lot, leading to some parts appearing very shiny compared to parts that are in shadow or at an angle off its usual reflective index.

That said I think you gave it a pretty good shot in this image. Just needs some thoughts on where it gets highlights in the future for the next time you decide to draw silk.