Priceless diamonds fall through the slushy interior of Neptune and Uranus and cover the solid cores of the giant planets, new research suggests.
The "diamond rain" forms as extreme pressures squeeze hydrogen and carbon more than 8,000 kilometres below the surface of the worlds.
Over thousands of years, the diamonds slowly sink through the icy layers to accumulate thickly around the central cores of both planets, experts believe.
These are not poor-quality gems - according to the scientists, the diamonds could reach millions of carats in weight.
In comparison, the famous Great Star of Africa, which forms part of the Crown Jewels, weighs 530.4 carats.
Neptune and Uranus are both known as "ice giants" because as well as having thick atmospheres, they are largely composed of ice.
For the new study, scientists simulated the extreme conditions found in the interior of the planets using a high-powered laser.
The laser created shock waves in polystyrene, a plastic made from a mixture of hydrogen and carbon, to produce the kind of pressures found deep below the planets' surfaces. The polystyrene simulated compounds made from methane, the hydrocarbon gas that gives Neptune its distinctive blue colour.
Nearly every carbon atom in the polystyrene was incorporated into microscopic diamond structures up to a few nanometres wide, the scientists reported in the journal Nature Astronomy.
Although the diamond formation lasted only fractions of the second, it was the first clear experimental proof of the theory.
Lead scientist Dr Dominik Kraus, from the German research laboratory Helmholtz Zentrum Dresden-Rossendorf (HZDR), said: "Previously, researchers could only assume that the diamonds had formed. When I saw the results of this latest experiment, it was one of the best moments of my scientific career."
The pear-shaped Great Star of Africa, set in the head of the Sovereign's Sceptre with Cross, is the largest segment of an original stone discovered in South Africa in 1905 known as the Cullinan Diamond that weighed more than 3,000 carats.
When Captain Frederick Wells, superintendent of the Premier Mine in Cullinan, South Africa, spotted the diamond during his daily rounds, he thought it was a shard of glass embedded in the rock wall as a joke.
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