I had try for weeks but I have no idea for this…
So… My intention is to make shape into this Anisotropic highlight with a function of Diffuse BSDF
It should work like a Diffuse BSDF as I mention:
a) it will disappear when there no light
b) it will rotate when the light move
BUT
c) it won’t change the thickness since I need to make it to sparkle shape
It should not be sees from downward. (This is not a need.)
I’d be grateful if you could help figure it out 
You can try this setup with your diamond mask and see if it gets close to what you need…
I only had a grey-scale image to work with ( not a B/W ) so the effect covers the whole band…
Unfortunately mine isn’t working. Maybe it’s because I used voronoi texture for the shape. But thank you anyway 
Then, could you share your file and we can see what we can do…
I’ll try to make a lite version of it since it is a private group project… I’ll send the lite version tomorrow or 2 days later.
new users can not upload attachments… what should I do right now?
Click on my Avatar and select Message…you can then send the file…
Better yet, in your messages you should have received a message from the Bot…There is a New User tutorial, go do it…it will raise your trust level and give you access to things like this…
