Not so far-fetched.
The journal, Nature Physics in a recent article reports that oceans of liquid diamond, filled with solid diamond icebergs, could be floating on Neptune and Uranus.
This research is based on first detailed measurements of the melting point of diamond and found that diamond behaves like water during freezing and melting, with solid forms floating atop liquid forms. The finding gives scientists a new understanding about diamonds and some of the most distant planets in our solar system.
Diamond is an incredibly hard material, tough to melt and thus its difficult to measure its melting point. When diamond is heated to extreme temperatures it physically changes from diamond to graphite. The graphite then melts into a liquid.
Ultrahigh pressures such as found in huge gas giants like Neptune and Uranus are some of the places where ultrahigh temperatures and ultrahigh pressures exist. In this study, scientists liquefied a small diamond weighing a tenth of a carat at by blasting it with lasers at ultra high pressures 40 million times greater than what a person feels when standing at sea level on Earth. From there they slowly reduced the temperature and pressure.
When the pressure dropped to about 11 million times the atmospheric pressure at sea level on Earth and the temperature dropped to about 50,000 degrees solid chunks of diamond began to appear. The pressure kept dropping, but the temperature of the diamond remained the same, with more and more chunks of diamond forming. Then the diamond did something unexpected. The chunks of diamond didn’t sink. They floated. Microscopic diamond ice burgs floating in a tiny sea of liquid diamond. The diamond was behaving like water.

With most materials, the solid state is more dense than the liquid state. Water is an exception to that rule; when water freezes, the resulting ice is actually less dense than the surrounding water, which is why the ice floats and fish can survive a Minnesota winter.
Up to 10 percent of Uranus and Neptune is estimated to be made from carbon.
Diamonds, Diamonds, Diamonds!
Bring the Bling!