https://en.wikipedia.org/wiki/Neptune#Internal_structure https://en.wikipedia.org/wiki/Neptune#Internal_structure > "At a depth of 7,000 km, the conditions may

https://en.wikipedia.org/wiki/Neptune#Internal_structure https://en.wikipedia.org/wiki/Neptune#Internal_structure > "At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.... the top of the mantle may be an ocean of liquid carbon with floating solid 'diamonds'."