Where and how can we mine diamonds outside of Earth but within our Solar System?

Question

Edited to give a reason why synthetic diamonds wouldn't work instead.

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In the distant future, all the diamonds are gone.

While synthetic diamonds are an excellent sollution for technology - as @ChrisJohns has noted, sometimes synthetic diamonds work best for processing, cutting etc - the jewelry industry has rejected them.
The natural mines have run dry, and laboratories will not satisfy the need for more.

Where in our Solar System can we find, and procure, more natural diamonds?

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Assume the following:

• Scarcity: Earth is basically out of accessible diamonds, and we can't dig any deeper than we have been.
• Price: Natural diamonds are so expensive, due to scarcity, that interplanetary mining is a viable option at this point to acquire more.
• Necessity: A replacement material will not be accepted.
• Naturally Occuring: Synthetic diamonds have, again, been rejected by the jewelry industry: people won't feel special if their jewelry has been made in an assembly line. Even if this isn't plausible, although it should be, humor me. Answer as if I'm right, it shouldn't affect where natural diamonds can be mined.

Answer Criteria:

• Abundance: The best answer will describe a location with a large amount of diamond (or maybe material to turn into diamond) that won't run out in at least twenty years of constant use
• Method: If you say "mine from Uranus because it has diamond rain" you must give a plausible explanation of how we can cut diamond at those pressures and then send it out into orbit. Yes, it's the future, but realism generally still applies.
• Specific Location: The best answer will not say "Mars has volcanoes and volcanoes have diamonds", it will say "go to this volcano and dig this deep"
• Most Economically Viable: While interplanetary mining is OK, and price of travel isn't a major concern, an answer describing mines on the moon will likely take precedence over mines on dwarf planets. Money will be spent, but less is best.

Answer 1

Diamond is the thermodynamically preferred state of carbon under conditions (pressure and temperature) in the Earth's mantle. On the surface they are thermodynamically unstable in theory, though in practice they will not decay with any sort of rapidity* under mundane conditions. (* By which I mean "lifetime of the universe".)

As such, the source of diamond will always have to be deep below the surface of a planet. The diamond needs to be carried from conditions where it is stable up to the surface or near-surface environment and then cooled quickly enough that it does not change into the preferred state (i.e. graphite) in its new surroundings.

Kimberlite pipes are the classic source of diamonds (not that every kimberlite pipe contains diamonds). When the igneous deposits are weathered away, the diamond (which is of course hard-wearing) can be carried away by e.g. water and end up in sedimentary placer deposits. So even though on Earth the vast majority is found in placer deposits, these placer deposits are not the ultimate source and we must follow the kimberlite. (See The Geology of Ore Deposits by J.M. Guilbert & C.F. Park, Jr. p. 347, 1987.)

With lower gravity applicable to every other solid body in the solar system, pressure increases less rapidly with depth than on Earth. This presents a unique problem given (as in the wiki article linked above) the extreme depth from which kimberlite source magma came.

Given all of the above:

• I expect diamond to be rarer on planets much smaller than Earth and Venus although they probably still exist.
• I expect diamond to be stable on the surface of Venus, given that it can withstand higher temperatures in that anoxic environment and the pressure actually helps.
• Wind speed on the surface of Venus is only ~1m/s (see Planetary Sciences, 2nd ed. by I. de Pater & J.J. Lissauer, p. 116, 2010). The density is of course higher than on Earth, in fact the atmosphere there is a supercritical fluid, and there are no free-running liquids like the rain we have on Earth. So I expect many diamonds to still be in or near their kimberlite pipes, if they exist.
• I am of the opinion that we simply don't know enough about the geology of Venus to determine whether there will be diamond-bearing deposits of kimberlite or an analogous rock. However, given just how common volcanoes are on Venus, my best guess is that they are significantly more common than on Earth.
• On Mars there may however be alluvial placer deposits formed when the planet had liquid water, which were fed by weathering of kimberlite and are therefore easy to extract, as on Earth. I don't expect them to be easy to find.

Venus wins in my opinion if you're willing to accept significant technological improvement. I don't know enough about the geography of the planet to name specific volcanoes but given their sheer number it should just be a matter of exploring the ones that most resemble kimberlite pipes.

However, without the various wide band-gap semi-conductors, efficient nuclear reactors, etc. required for industrial-scale robotic mining on the surface of Venus, I would look for placers on Mars.

Answer 2

Actually synthetic diamonds are already widely used, especially for industrial applications and are cheaper than mined ones. According to this article 97% of industrial diamond is synthetic.

Currently the only real difference is that it is difficult to make the sort of large stones needed for jewellery etc but even then a lot of the costs come from cutting and processing them rather than genuine scarcity.

Bear in mind that this is a slightly different situation to something like gold which has both industrial value and aesthetic desirability because the desirability of a gem diamond comes from its size and quality ad you cant just melt down a handful of industrial diamonds to make a big gem.

It is also likely that while diamond has properties which would be useful for processors etc these sort of applications would require thin films which would be much better made by industrial processes like vapour deposition. Indeed it is much more plausible that these sorts of technologies are only possible with synthetic diamond.

Similarly consider that the silicone chips are expensive not because of the scarcity of the raw materials but because of the technical sophistication of the manufacturing processes and a diamond is no closer to being a chip than a handful of sand is.

So it doesn't really make sense that natural diamonds are better suited to these sorts of applications.

Answer 3

Neptune and Uranus have gigantic supplies of diamonds in their mantles and cores.

The extreme pressure and temperature deep within Uranus may break up the methane molecules, with the carbon atoms condensing into crystals of diamond that rain down through the mantle like hailstones.[64] Very-high-pressure experiments at the Lawrence Livermore National Laboratory suggest that the base of the mantle may comprise an ocean of liquid diamond, with floating solid 'diamond-bergs'. (Uranus - Wikipedia)

A likewise scenario is likely to exist on Neptune, too.

The mining would have to be done by robots, drilling into the core of these ice giants, utilizing nuclear bombs. In case their diamond supply is required really badly, the atmosphere of these gas giants would have to be thinned out enough to soften the conditions enough for a drilling operation to be carried out. This requires removing at least 60% of the atmospheric content on the planet in question.

Another option would be to blow the planet to pieces, so that the frozen diamonds can be collected from the resulting space debris. While collecting the diamond chunks would be easier this way, it is simply impossible to break up a planet (let alone a giant planet!) with our modern technology. We would definitely require weapons of extreme destruction, such as gigantic antimatter missiles, which contain antimatter warheads in the mass ranges of kilograms! For a comparison, the mass-to-energy conversion for littleboy and fatman were no more than a few grams.