A simple, low-poly model of a Tapered Baguette-cut diamond. The material is meticulously made using Blender's shader nodes, and provides a perfect mix of color dispersion, giving it the diamond's "fire" effect, as well as light reflection, or "glint" in Eevee.
The shader works flawlessly both with Eevee and Cycles renderer, as I've showcased in different render variations of the gem.
For people who are handy with material nodes, dispersion ratio, index-of-refraction and roughness (reflectivity) of the gem are all up to you to modify at your pleasure.
The renders in the scenes use a HDRI which was created by Andreas Mischok and downloaded from Poly Haven under a CC0 license, and I've included it in the package if you want to keep using it yourself.
Added .fbx and .obj exports as well for your convenience.