what precious material rains on neptune and uranus?

On Neptune and Uranus, it rains solid diamonds, also known as "diamond rain". These diamonds are formed deep within the planets, where the extreme pressure and temperature conditions cause the carbon and hydrogen in their interiors to crystallize into diamond structures. The diamonds are believed to be small, with nanometer-scale lengths, but over thousands of years, they could slowly sink through the planets icy layers and accumulate as a thick layer around the solid cores.