While still busy with a lot of other projects at the moment and a little bit stuck with the special tutorial on space / spaceships / galaxy backgrounds and the likes – here’s a quick requested tutorial. 
I tried to make it as simple as possible, yet look somewhat convincing. 

One very helpful functionality for this tutorial is the snapping of nodes [%]. It allows you to snap nodes to other shapes [their nodes as well as their lines]. This makes it a lot easier to adjust the shapes we will create to stay inside with the facets of the diamond. 

This is the quick and dirty way. The lights and shadows of a diamond are somewhat 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: