The atmospheres of Uranus and Neptune and Saturn and Jupiter contain methane (about 15% of the atmospheres of Uranus and Neptune are methane, and the atmospheres of Jupiter and Saturn contain no more than 0.5%). Strong lightning can turn methane into carbon in the form of coal ash. The coal ash solidifies into graphite in the process of sedimentation, and then turns into diamonds under the action of high temperature and high pressure. This is the origin of diamond rain.
For example, Neptune's atmosphere is completely different from our earth's atmosphere. Its bottom is very close and hot, while Saturn and Jupiter are gaseous giant planets with a radius of about 10 times that of the earth.
Diamond rain on these planets begins in the upper atmosphere. Lightning breaks down methane and turns it into carbon black (carbon). As the carbon black decreases and the air pressure increases, it turns into graphite again. After another 4000 miles, the pressure increases to turn graphite into diamonds. Diamonds continue to fall about 20000 miles (no one knows how deep Saturn or Jupiter's core is) - almost three times the diameter of the earth - to their core. At this time, the pressure and temperature may have been high enough to turn the diamonds into a vast ocean of liquid carbon - the diamond sea.
The diamonds on these planets are "Diamond rain" in the process of falling. When they sink to the bottom, they are likely to form a "Diamond sea" due to the influence of high temperature.
How big is the largest diamond ever found in the world? Maybe you'll guess it's the 530 carat African star? In fact, the largest diamond is probably not on earth, but in heaven. Scientists speculate that a white dwarf named "bpm37093" has slowly turned into a 10 billion star × 1 trillion × 1 trillion carats of earth sized "diamond" superstar!
The internal density of this white dwarf is very high. The density of carbon crystal structure formed by crystallization is millions of times higher than that of diamonds on earth. It can be described as a compact super diamond.