"It's so far down that we cannot observe that region of the atmosphere with spacecraft," Delitsky said.
The cores of Jupiter and Saturn would melt diamond, but the centers of Uranus and Neptune are cold enough to keep it solid -- a relatively chilly 6,000 Kelvin (10,340 F).
Delitsky envisions that there could be "hand-sized diamonds" on these planets, although she has not performed those calculations. There could be as many as 10 million tons of diamonds produced from lightning-generated soot on Saturn, the researchers said.
Are they really there, and can we get them?
Scientists not involved in the project expressed a mixture of interest and skepticism.
"This kind of study shows that we are still far from understanding the composition and structure of Jupiter and Saturn, two planets which are respectively about 300 and 100 times the mass of our Earth and were crucial players when the whole Solar System formed," said Tristan Guillot, planetary researcher at the Observatoire de la Cote d'Azur in France, in an email.
If there are diamonds, they likely wouldn't be large iceberg-sized "diamondbergs," Guillot said. Any such structures would fall rapidly toward the planet's interior, depleting the outer layers of carbon. That would result in a carbon-poor atmosphere -- which, scientists know, is not the case.
It's plausible that carbon in the form of diamond could be stable in the interiors of Saturn and Jupiter, says Peter Read, professor of physics at the University of Oxford, but he's skeptical that diamonds like those on Earth would be found. Rather than chunks of precious stone, there may be instead clouds of condensed diamond material, he said -- but we just don't know.
And if there are diamonds for the taking, they'd be in regions in the deep interiors of these planets that are challenging to reach, Read said. The temperatures are enormous, with pressures up to a million times what's observed at the surface of the Earth.
This brings up important questions for would-be diamond miners: What would the mining robot be made of in order to withstand such temperatures and pressures? (Delitsky suggests diamonds!) How would the spacecraft be powered?
"There are probably many easier (and cheaper!) ways to obtain diamonds than going to Uranus or Neptune in a heavily reinforced and insulated submarine!" Read said in an email.
Although no diamond-mining space missions are on NASA's docket, we may know more about the likelihood of diamonds soon. The Juno spacecraft will arrive at Jupiter in 2016, and the Cassini spacecraft will dive into Saturn in 2017, collecting information about the gravity fields and magnetic fields of these planets.
These spacecraft won't get deep enough to explore the theoretical diamond regions, but they will at least "be able to check whether we detect signatures of density changes that would occur near where diamond is predicted to form," Guillot said.
More ambitious explorations of the outer planets may help us determine if these theories about diamonds will last forever.