I just start with a black base coat, paint most of the gem with a dark purple that I just mix from purple + black, paint purple in a "J" shape down the side of the gem, mix in a bit of white to the purple to create a light pink and do a fine line over the bottom half of the J and then finally do a teeny tiny white dot or thin line near the top but not directly on the J itself so it looks like light is striking that part at reflecting directly. Finish it off by applying a gloss varnish.
I do the blue gems the exact same way, but using blue instead of purple. The only thing I find with the purple is to avoid making them look too pinkish you want to make the "body" of the gem slightly darker so that your "J" shape can in turn be slightly darker, that way the pink is only the most extreme highlight on the J and doesn't overwhelm it.
Make sure you use a good brush to get those super fine lines.