According to BBC
, scientists say that data for the planets indicates that carbon is in its crystal form. Methane is turned into carbon due to lightening storms, and then it turns into chunks of graphite as it falls down to the surface, and then it turns into diamond.
According to ExtremeTech
, the diamonds will then fall for thousands of miles until they reach the planets' cores. It is believed that the diamonds turn into liquid carbon due to the pressure and temperature.
According to Huffington post
, the theory of diamonds being able to form on the planets are not only based on the fact that there is methane in the planets' atmosphere, but also that the temperature is high enough for them to form.
For the longest time scientists have believed that diamond rain may occur on Uranus and Neptune, but they were not sure if they could say the same thing for Jupiter and Saturn, due to the planets having lower temperatures and a smaller proportion of methane than Neptune and Uranus.