Diamonds, big enough to grace a necklace or bracelet, may be raining down in the atmospheres of Saturn and Jupiter, U.S. scientists say.

Carbon in its dazzling crystal form may be abundant on the solar system's two largest planets, researchers told the annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Denver last week.

The diamond creation starts when lightning storms high in the atmosphere turn methane into soot -- carbon -- which as it falls into layers of the atmosphere with increasing pressure hardens into chunks of graphite and then diamond.

The biggest of the diamonds would be more than a quarter of an inch diameter, "big enough to put on a ring, although of course they would be uncut," Kevin Baines of the University of Wisconsin-Madison and NASA's Jet Propulsion Laboratory said.

Baines and colleague Mona Delitsky from California Specialty Engineering analyzed the latest temperature and pressure predictions for the planets' interiors, as well as new data on how carbon behaves in different conditions.

Stable crystals of diamond would "hail down over a huge region" of Saturn in particular, they concluded.

Other researchers said the findings, based on what is known of the atmospheres of Jupiter and Saturn, make sense.

"The idea that there is a depth range within the atmospheres of Jupiter and [even more so] Saturn within which carbon would be stable as diamond does seem sensible," Planetary scientist Raymond Jeanloz of the University of California, Berkeley, told the BBC.