The experiments were conducted by squeezing methane inside an "anvil" and heating it with a laser beam. The diamond crystals produced are only about 10 micrometers long -- smaller than the diameter of a human hair. Benedetti says it is a "close call" as to whether the diamonds can be seen with the unaided eye.

But the diamond crystals inside the planets might be "much bigger," she notes, although no one knows how big. Possibly, numerous crystals combine to form boulder-sized objects.

Theorists had suggested that diamonds might form inside Uranus and Neptune, but computer simulations had predicted this would occur only near the center of the planets. The new experiments indicate that diamonds could form under a broader range of conditions.

For instance, the experiments recreated pressures believed to exist about 7,000 kilometers (4,340 miles) below Neptune's cloud tops -- only one-third of the way to the planet's center.

Indeed, Benedetti believes, diamonds may be common enough in Uranus and Neptune to have important effects on the planets' overall temperatures and magnetism. Being denser than their surroundings, the diamonds would sink, losing their gravitational energy in the form of heat. This might explain why Neptune loses more heat than it absorbs from the sun.

However, the diamonds are unlikely to have much impact on jewelry markets, given the harsh environment where they are stored. "Nothing we could build would be able to withstand it and get back," says Benedetti.