Even Bob Dylan never imagined a hard rain like this.
The ice giant planets Uranus
(left) and Neptune
(right) differ from the gas giants Jupiter and Saturn in composition; they contain mostly "ices" of water, ammonia, and methane. And that makes for interesting planetary interiors: In the mantles of Neptune and Uranus, high temperatures probably break methane into its components of hydrogen and carbon. Astronomers think that intense pressures may then squeeze the free carbon into crystalline latices, aka diamonds. As Baker and Ratcliff explain: "Diamond hail, as small as salt grains or as large as boulders, may steadily rain through the liquid mantle and pummel the rocky core. The core may be covered in a thick layer of diamonds, more massive than any diamond mine on Earth."
So far, the diamond rains of Uranus and Neptune are theoretical, and planetary scientists say they need more data before they can determine whether or not this bizarre phenomenon actually occurs. Unfortunately, no spacecraft are currently scheduled to go and explore these remote worlds.