Latest attempt at an opal effect, Cycles, 64 samples with compositor denoising in 3.0.0 alpha 2021-09-01 16:47. The inner flakes are a distorted cube, distributed via geometry nodes (method here, thanks @Xeofrios), with a simple iridescent material. The containing object material is a Principled BSDF shader with both Transmission and Transmission Roughness at 1.000.
OpalTest017.blend (1.5 MB)
I’m on a laptop underpowered for this, if anyone wants to try improving it or just play with it please feel free.