https://blendermarket.com/products/fake-diamonds-shader

This product is a collection of 4 diamond shader node groups to let you quickly and easily make a lot of diamonds. These node groups are very easy to use, just use them as shaders, put in the UVMap and you're good to go. The  diamond shaders are, as the name suggests, fake. They are a kind of cheat that mimic the refraction of diamonds but does not actually refract the light. This method renders very quickly, compatible with both Cycles and Eevee, and produces a believable diamond look. You don't have to wait forever to compute all the refraction and reflection of real diamonds.