Acadia I just downloaded that prop and I'm so sorry to tell you but you're wasting your time.

That tiara doesn't have the shapes of real diamonds at all. The back is flat instead of having a cone shape made of facets. And the front of the stones only have 7 facets - a real diamond has over 50 facets on the front and without those facets any "physics" based simulation of light is not going to make them look like diamonds. They're going to look like flat pieces of bevelled glass.

I noticed that the prop actually comes with a diamond photo texture - I suggest you use it for this prop. If you like, you can still improve the original as I've done here. A proper specular highlight size, and some fresnel effect reflection helps improve them - I think this looks fine.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)