Summing things up
Ultimately, this is the quick and dirty way. The lights and shadows of a real diamond are a lot more complex.
If you want to go more realistic, check out reference images. Look at the way the light breaks and adjust the white gradient shapes. Some might need to darken the stone rather than lighten it.
I hope you have fun with this and maybe try out some other stone cutting shapes. Remember google images is your best friend when it comes to finding reference images or just to check if your idea of how it should look resembles the real thing.
I hope you enjoyed this one as much as I did finally writing something useful again.
Here is a quick test with a little bitmap magic, added layers for shine, a bit of star-burst and smaller sparkles.
Here’s the download file: