Hello organisms; look at your planet, now back to Jupiter, now back at your planet, now back to Jupiter. Sadly, it isn't Jupiter, but if it stopped orbiting so close to the sun and accreted a massive atmosphere of volatiles, it could look like Jupiter. Look down, back up, where are you? You're in a planetary system with a planet your planet could look like. What's in your anoxic marine sediments? Jupiter has it; it's methane produced, in this case, by photochemical reactions rather than by biological processes. Look again--the methane is now diamonds. Anything is possible when your planet is a Jovian gas giant with core pressures sufficient to metallize hydrogen and not a large-ish rocky planet with surface conditions near the triple point of water. I'm on the Earth.
In 2061 Odessy Three, Arthur C Clarke referenced Jupiter having a diamond core. A plot point had a chunk of the former Jupiter's core embedded in Europa. Pretty cool.
There's no reason to believe soot would clump in Jupiter or Saturn's atmosphere. It would disperse and never become diamond. There is probably plenty of diamond on both planets, but that isn't how.
/humor /irony /labor_exploitation_commentary