On Saturn and Jupiter, lightning storms turn methane into carbon. As this falls, the carbon hardens into chunks of graphite, and then diamond. At about one centimeter in diameter, they are definitely big enough to make a diamond ring.
Although scientists can’t actually go and observe this spectacular rain, they are very certain that Saturn creates about 1,000 tons of diamonds a year. And this diamond creation process is not a short one. The methane turns into graphite after falling about 1,000 miles. And after turning into diamond, they continue to fall the length of about two and a half Earth spans. Once they get into that extreme of depth, it’s unsure what happens to the diamonds, although it’s unlikely they remain solid.
At the extreme depths of the planets, it’s possible a “sea” of liquid carbon is formed from the diamond rain. Uranus and Neptune are much colder at their cores than Saturn and Jupiter, so diamonds could last forever there.