ThreeJS diamond reflection and refraction

Im experimenting with three js and trying to make diamond. I found a couple of examples with shaders and without but what cache my curiosity is this sample from sketchfab. Its seems that its not using shader but there is some trick how diamond look is achieved, like you using backside of diamond texture while moving camera and adding other effects, or maybe im wrong.

https://sketchfab.com/3d-models/gems-92ff52fe0f2e44bc877c133191df7195

I tried to render first backside of diamond with cubemap environment and use rendertarget cubecamera, then i render second scene where im using previous scene as a texture with reflection and refraction but no results so far.

Does someone have any idea how this can be done, i would really like to hear your opinions how you would bulid this? Thanks.