According to the search results, it is believed that diamonds rain on both Uranus and Neptune. The diamond rain phenomenon is caused by the immense pressures that occur deep in the atmosphere of these planets. Methane, which is present in the atmosphere of these planets, has carbon in it, and that carbon can occur by itself and also be crushed by the immense pressures that happen deep in the atmosphere. Inside the planet, when it gets really hot and really dense, these diamonds form and accumulate, and then they become even heavier. And that means that they kind of rain down in the atmosphere. On Saturn, it is also believed that it literally rains diamonds.