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Uranus and Neptune May Have "Oceans of Diamonds"
Third Position writes "Oceans of liquid diamond topped with solid 'icebergs' of the precious gems could be on Uranus and Neptune. The first-ever detailed research into the melting point of diamond found it behaves like water during melting and freezing — with its solid form floating on the liquid. A large diamond ocean on one or both of the planets could provide an explanation for an oddity they both share: unlike Earth, they do not have magnetic poles that match up with their geographical poles." The article doesn't mention what the pressures might be like in these outer-planets environments, but the researchers found that liquefying diamond requires 40 million times Earth's atmospheric pressure at sea level.
The possibilities of exploring the outer "ice giants" is massive. I think, at least. I may not even make the pun because I think the idea of exploring them is so interesting.
Submarines are designed to handle a test depth of maybe 1600 ft which means maybe 50 bar of pressure. At that pressure, the atmosphere of Uranus is a little below freezing. The gravity is less than Earth. I suspect that with correct ballasting you could make a metal sphere float in the atmosphere for quite some time by keeping the insides pressurized to a convenient atmospheric pressure. So sticking around for a while isn't hard.
I can't find any good information on the radiation environment there and if you could put humans in the little bubble circling Uranus.. um.. yeah, I lied above.
Gentoo Sucks
Hal Clement thought too small. Mesklin may be too low pressure for complex life.
One of the reasons earth is so amenable to life is that ice floats, so the oceans remain deep and liquid. The hydrocarbon oceans of Mesklin would be shallow and cold, a thin layer of liquid ammonia or methane over ices and clathrates. Thus they wouldn't serve as a moderator of temperature and reservoir of life the way Earths oceans have.
But if life based on crystalline carbon at millions of atmospheres is possible at all, it's all the more possible if the carbon-cycle resembles the water cycle on Earth.
Synthetic diamonds are for the most part, industrial grade which tends to be opaque unlike gem quality natural diamonds which are transparent, contain Nitrogen and don't fluoresce under UV like synthetic diamonds generally do. Synthetic diamonds are synthesized in rapid fashion which leaves two major crystal phases in the finished material which is responsible for the fluorescence under UV light. Any transparent synthetic diamonds tend to either be devoid of Nitrogen (crystal clear) or have a yellowish tinge to them caused by Nitrogen in the crystal. Natural diamonds have Nitrogen in them but they form in such long periods of time that there is only one major crystal phase in them and the Nitrogen has migrated to regions in the crystal in such a way as to leave the diamond clear instead of yellow. So yes diamonds can be synthesized cheaper than those dug out of the ground. However, they are not quite the same as of today's technology and can often be differentiated from natural diamonds because of minute differences in their characteristics.
Sigs are too short to say anything truly profound so read the above post instead.
My guess is that the difference in density may be strongly dependent on the pressure. At standard conditions, diamond is actually the densest form of pure carbon, and at atmospheric pressure, carbon sublimates instead of melting. It seems possible to me that liquid diamond is more compressible than solid diamond, such that the liquid density is more variable than the solid density with respect to pressure. Under a relatively low applied pressure (well, still gigapascals), diamondbergs would sink. At some phenomenal pressure, the densities would match and solid would be neutrally buoyant in liquid. Above that pressure, the atoms in liquid diamond would be more crushed together than those in the diamond lattice, and the crystal would float. The inherent strength of the cage-like solid diamond structure makes it energetically favorable, despite the atoms being farther apart.
"FDA staff reviewers expressed concern about the number of patients who were left out of the study because they died."