I tried various transparent meshes, like icospheres, prisms and some gemstones. Then I chose the most "classic" one.
Even though the "dispersion" is enabled, I couldn't really see rainbows... So I added a fresnel texture to diamond's internal volume using Cauchy's equation.
Now the white light splits into rainbow properly, but the rendering is much slower (need a lot of spp to get a clear picture). Both diamond renders have 150spp