An atmospheric data for the gas giants indicates that carbon is abundant in its dazzling crystal form. Lightning storms turn methane into soot (carbon) which as it falls hardens into chunks of graphite and then diamond. These diamond "hailstones" eventually melt into a liquid sea in the planets' hot cores.
It all begins in the upper atmosphere, in the thunderstorm alleys, where lightning turns methane into soot. As the soot falls, the pressure on it increases. And after about 1,000 miles it turns to graphite, the sheet, like the form of carbon you find in pencils.
By a depth of 6,000km, these chunks of falling graphite toughen into diamonds strong and unreactive. These continue to fall for another 30,000km. Once you get down to those extreme depths, the pressure and temperature are so hellish, there's no way the diamonds could remain solid.
The biggest diamonds would likely be about a centimeter in diameter, big enough to put on a ring. The bottom line is that 1,000 tonnes of diamonds a year are being created on Saturn.