Based on research, scientists suspect that diamonds fall from the atmosphere like rain on Neptune and Uranus. The conditions are right for this type of phenomena, such as atmospheric pressure, intense heat and carbon atoms in the atmosphere.
Researchers note that Neptune's atmosphere contains high levels of methane. Due to the extreme temperatures of between 5,000 and 12,000 degrees Fahrenheit, the methane molecules can potentially separate into hydrogen and carbon atoms; if the carbon succumbs to the intense pressure, it can turn into diamonds. The atmospheric pressure on Neptune can be 200,000 and 6,000,000 times what the pressure is on Earth.