Draw your diamond with a pencil. Use a ruler to help you make straight lines. This makes the facets look more real. Add ink and erase the initial pencil lines.
Draw light, thin strokes in ink on the side of the diamond from which the light is coming. The light will hit on the opposite side of the diamond, so do not shade as much there. This gives the illusion of both faceting and depth.
Use a white gel pen to add some highlights to the facets directly facing your light source. Draw over the ink.
Add some small white dots of white gel pen on the lines that form the diamond, breaking the lines up. This makes the diamond appear to sparkle.
Outline the entire diamond in white ink for extra emphasis, if desired.