Maybe even on Uranus.
But we are in the realm of hypotheses, and these showers have been discussed for 40 years. After all, they are distant planets, and only one probe has ever taken a close look (5,000 km from Neptunian North Pole was its closest approach), Voyager 2.
And it didn't see any diamond rain.
However, there are also telescopes and computers that run simulations and even experiments. For example, the hypothetical Neptunian diamonds have been recreated in the laboratory.
Neptune and Uranus are gas giants, meaning their outer layers are composed of gas. Both planets contain a lot of hydrogen and its derivatives , such as methane, water, and ammonia. It is precisely methane that gives Neptune its stunning blue color.
These elements freeze; Neptune, beneath its 3,000 km-deep gas layer, has a layer of ice over 17 km deep. This ice is strongly compressed by the upper layer and heated by the layer just below, thus becoming a dense, hot liquid.
Carbon and hydrocarbons, also present, could be so compressed at these incredible pressures that they have assumed the structure of a diamond.
Being heavier than methane or hydrogen, these diamonds sink toward the center of the planet, fusing to form gems several meters in diameter!
Other models, however, suggest temperatures such that the carbon, instead of structuring like a diamond, passes into a liquid or semi-liquid state.
Regarding rain, it's hypothesized that continuous lightning strikes in the atmosphere break the bonds of carbon, leaving it suspended in suspension. However, given the extremely high pressure, this carbon sinks into the atmosphere, where the incredibly high pressure and temperatures, just as in the deepest layers, could transform it first into graphite and then diamond.
In that form, it falls like raindrops.