Hey Diana.
The react three fiber link that I’ve sent is using MeshTransmissionMaterial with some settings to make the diamond more realistic. It is pretty simple to use.
But those others are using WebGI, a proprietary renderer created by some friends that is threejs based but it is unique. Basically, they created an exclusive version of threejs and extended it with their deep knowledge of very advanced computational techniques. With that, they were able to create a post-processing pipeline that is much better on performance than anything that I’ve ever seen. We can add more than 40 post-processing passes without losing any performance. Specifically talking about the diamond, it is a custom material that calculates the internal refractions 9x more accurately than the other diamond shaders and yet is capable of getting a very good performance. Not to mention that SSR, SSAO, and SSBEVEL are applied to make the results pop. We also have a very good DOF PP pass that is way faster than Evee DOF and works in real-time in the browser. And all of that works even better on mobile.
So, I must say that if you need ultra-realistic results for diamonds with no impact on performance, it must be webgi. I don’t know any other solution available capable to provide what this tool can.
I hope this can be helpful to you.
Cheers