It rains diamonds on Neptune and Uranus
The extreme pressure and temperature conditions in the interiors of Neptune and Uranus — millions of atmospheres of pressure at temperatures of thousands of degrees — are sufficient to break down hydrocarbon molecules into carbon and hydrogen. The carbon atoms, under these conditions, form diamond crystals that literally precipitate downward through the planetary interior like rain. Scientists have experimentally replicated these conditions and confirmed diamond formation. On Neptune, it may rain literal diamonds into a liquid diamond ocean.
What we consider one of Earth's rarest and most valuable materials falls as precipitation on distant planets. The universe is neither scarce nor romantic about diamonds.
“It literally rains diamonds on Neptune and Uranus. There may be liquid diamond oceans deep in those planets.”