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In this publication from 1981, was shown that Neptune is raining diamonds:

Many of the current models of Uranus and Neptune postulate a three-layer structure, consisting of an inner rocky core, a middle ‘ice’ layer of fluid, $\rm H_2\rm O$, $\rm{CH}_4$, $\rm{NH}_3$ and an outer hydrogen–helium layer of solar composition. The estimated pressures and temperatures of the ice layer range from about 6Mbar and 7,000K at the inner core-ice boundary to $\approx$ 0.2 Mbar and 2,200K at the outer ice/hydrogen–helium boundary. I point out here, that shockwave experiments on these liquids, as well as theoretical studies, imply that the $\rm H_2\rm O$ and $\rm{NH}_3$ in the ice layer are almost totally ionized, and the $\rm{CH}_4$ has been pyrolysed to carbon, possibly in the metallic or diamond form.

Question: In the decades since this work was published in 1981 have these predictions been addressed further, or even proven or disproven?

If this turns out to be true, would these diamonds have different physical properties than those on Earth because of the different formation mechanism?

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    $\begingroup$ BTW, space travel is expensive. If there were gold bars just sitting around on the Moon, it wouldn't be profitable (with current technology) to send a ship to collect them. $\endgroup$ – PM 2Ring May 3 at 9:39
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    $\begingroup$ As we know, Neptune is raining diamonds I don't ;-) Can you back this up with a link? $\endgroup$ – Jan Doggen May 3 at 11:50
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    $\begingroup$ What question are you asking? Carbon-diamond is a very specific material. Are you wondering about quality, such as structural defects, contamination? Are you aware that Terran diamonds vary in quality -- not to mention that we can grow them in labs these days? $\endgroup$ – Carl Witthoft May 3 at 13:23
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    $\begingroup$ @CarlWitthoft It's even In Googles autocomplete ;-) Still, I haved edited in Marvin Ross' original article. $\endgroup$ – Jan Doggen May 3 at 13:23
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    $\begingroup$ After several edits, I think the question is fine now. Once you get an answer, if it is positive you could consider asking a new question in Space Exploration SE, perhaps something like "What might a mission to collect Neptunian atmospheric diamonds be like? What are the technical challenges?" $\endgroup$ – uhoh May 4 at 2:35
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People have demonstrated high-pressure diamond formation using laser-driven shocks, much in line with the original idea. The diamond rain idea seems to be doing well (popular science article about it, with references).

However, it is quite likely that the diamonds could have different properties because of formation mechanisms. There are a lot of unusual allotropes of carbon, and local conditions of formation (not to mention impurities) may cause the carbon to turn into these forms rather than the standard diamond lattice.

Simulations show that beyond 1000 GPa diamond has a BC8 structure. However, this is a pressure unlikely to occur inside Neptune's atmosphere: it occurs deep in the mantle, so sinking diamonds may shift crystal form.

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There are probably a variety of factors. How fast they form, how fast they sink through the atmosphere. Like rain, there's likely a part in the atmosphere where they form and grow, and a part, deeper in the atmosphere where the temperature gets too high and the diamonds dissolve again into plasma, perhaps releasing some free/charged carbon to return upwards and be part of the cycle all over again.

If I was to guess, I would guess that the diamonds formed in gas giant atmospheres would be much more impure. The interior of the Earth is relatively slow moving and consistent in temperature, often cooling very slowly. It's the same process which helps form veins of metals form, which are followed in mining. This very gradual cooling in Earth's core probably plays a role in the purity of diamonds. Earth's core also has very little nitrogen, Diamonds formed in Earth are about 99.95% carbon atoms on average, with nitrogen, often the primary impurity.

https://www.gia.edu/diamond-description

http://www.dianerdiamonds.com/diamond_info/nitrogen_in_diamonds

The relative abundance of nitrogen available in gas giant planets suggests to me that on average, gas giant diamonds would be more impure. Less consistency in temperature during formation and an availability of nitrogen suggests to me that 99.95% pure carbon diamonds are unlikely. There's also the problem of how large they'd be. If diamonds form slowly, which seems likely and as they grow larger, they would have a density problem, likely slowly falling deeper into the gas giant's atmosphere, where they would get too hot and likely become plasma and dissipate.

If I was to guess, I'd guess that the diamonds that form inside the 4 gas giant planets in our solar-system are more along the grain of sand to perhaps raindrop size and probably quite impure. But I'm mostly guessing. There might be a planet out there somewhere, (perhaps a mini Neptune) with just the right mix of lower gravity and possible suspension and temperature where it forms grapefruit or basketball sized diamonds of high purity. It's certainly possible, but my guess is that diamonds inside our solar-systems 4 gas giants would be pretty uninteresting, though the chemical and gas-giant geological processes that form them would still be interesting, but I don't think the diamonds would stand out as gem quality or worth harvesting.

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