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The People of The Shining Isles have long been exploited. They are cruelly used in mines, excavating tonne after tonne of rock and dirt in pursuit of what the Warriors from Away call ‘diamonds’. These small, shining rocks are apparently of great value to the invaders for some reason or another, though the People have always ignored them, preferring to grow vegetables to sacrifice in order to prevent the ancient Smoking Gods from waking.

The question here is pretty simple: Is there a set of circumstances that can lead to high concentrations of diamonds on an otherwise volcanic island chain (think Hawaii), or will the production of such islands always act to destroy the gemstones before they can be mined?

The length of time required is of no consequence (this is an old world), and the number/distribution of tectonic plates is similarly unimportant. The world does have to be earthlike though, so keeping gravity/atmospheric composition the same would be appreciated. I’m open to ridiculously unlikely chains of geologic events leading to this state, but in deference to William of Ockham simpler answers are better.

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Diamonds are formed by crystal growth of carbon in suitable conditions, therefore you need to have:

  • suitable pressure and temperature
  • suitable chemicals (carbon to begin with)
  • time to allow crystal growth

This usually means that you have an intrusion of magma deep underground, which slowly cools down, and then it is lifted closer to the surface by either tectonic or erosion.

With an active volcano that magma would diffuse to the outside rather quickly. Unless diamonds have already formed in there, and can withstand the sudden change of conditions (unlikely, hot lava, oxygen and carbon mean an expensive bbq bed), you cannot have diamonds.

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  • $\begingroup$ I agree the problem is not getting diamonds it is getting them anywhere near the surface. That said a volcanic island does not need to be on volcanic crust, see iceland. $\endgroup$ – John May 29 '19 at 19:42
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As has been mentioned already, your volcanic island is not a typical environment to find diamonds. So if the diamonds formed in an early geological formation, that later eroded and cast the diamonds into a strong current where they lay on the seabed. They could reasonably get pushed up by the forces that produced the volcanic island chain.

If this sounds too implausible for you since it depends on a very specific occurrence of unlikely events, you might consider having other valuable materials on the island that do occur with volcanic activity

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If you want diamonds, you pretty much want kimberlite pipes.

If you want kimberlite pipes in your islands, you might want the island of Malaita, in the Solomon Islands.

I don't know if there are actually any diamond on Malaita, but it looks vaguely-plausible enough from here.

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    $\begingroup$ They aren't kimberlites, as the author himself noted in another paper: "Despite containing a mantle xenolith assemblage similar to that found in kimberlites, the host Malaita rocks are mineralogically and geochemically different from kimberlite." There was a report in 2000 that they'd identified sub-0.1mm microdiamonds, but in the 19 years since then, nothing. And there's been no rush to look for diamonds on islands and no paradigm change in exploration. $\endgroup$ – Keith Morrison May 29 '19 at 20:01
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  1. Meteorite comes!

  2. Meteorite slams into carbony crust. Impact diamonds are formed, like those in Poigai crater.

  3. Also meteorite itself was full of space diamonds so doubly diamonded.

  4. Meteor hit hard. It made a volcano happen right there.

meteor volcano https://www.newscientist.com/article/dn3171-earths-volcanism-linked-to-meteorite-impacts/

  1. Volcano carried up a mix of space diamonds and impact diamonds.

Fortunately no-one but microbes was around for all of that. Later on things settled down and your islanders moved in.

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    $\begingroup$ Yeah, no. First of all, you're talking billions of years. The odds of a volcanic island surviving billions of years and still being volcanic are roughly (takes out calculator) zero. The odds of it still being an island are roughly, um, zero. The oldest surviving oceanic crust is only 340 million years old: any volcanic islands older than that are either subducted or plastered on to a continent. $\endgroup$ – Keith Morrison May 29 '19 at 2:05
  • $\begingroup$ @KeithMorrison Could this work with a more recent (and presumably smaller) impact? $\endgroup$ – Eth May 29 '19 at 11:59
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    $\begingroup$ (1/2) On a practical level, not really. Impact diamonds are primarily microdiamonds (ie, diamond dust). As an example, Popigai is considered the world's largest diamond deposit, the result of an impact that left a 100 km crater (it's currently tied 7th largest known on Earth) and in area that had a graphite-rich gneiss at just the right distance and depth from the impact. The problem is the largest of those diamonds are under 2 mm: the graphite was converted so fast due to the transient pressure and heat that it didn't have time to slowly grow the large crystals that make gemstones. $\endgroup$ – Keith Morrison May 29 '19 at 14:58
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    $\begingroup$ (2/2) So while Popigai has a huge amount of diamond mineralization, it's not in any form what people think of when they think "diamond", no more than you'd think of a masonry saw being "diamond" just because it's coated in industrial diamond abrasive. It's not even economic; it's cheaper to grow industrial diamond than mine it. Raw diamond isn't particularly more attractive than other crystals; it's the cutting and polishing that makes it look spectacular. So really, the best you'd get is something that might be really abrasive and somewhat sparkly sand. $\endgroup$ – Keith Morrison May 29 '19 at 15:07
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Yes, if you are flexible with what you call a volcanic island.

An island on continental crust could have diamonds one on oceanic crust will not. You can have islands with volcanoes that have diamonds but the volcano would not be responsible for the island. you need a island like iceland A chunk of continental crust that has been moved far from the mainland. but unlike iceland you want the island on a subduction zone more like japan. This means your island will be big Iceland is probably the lower limit for the size of an independent portion of continental crust without so truly unique circumstances. large enough they are unlikely to have a single political structure.

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  • $\begingroup$ Iceland does not sit atop continental crust; Iceland is a bit of oceanic crust which has been lifted above sea level. $\endgroup$ – AlexP May 29 '19 at 20:05
  • $\begingroup$ @AlexP no there is continental crust pnas.org/content/112/15/E1818 $\endgroup$ – John May 29 '19 at 20:26
  • $\begingroup$ OK, so a little bit of Iceland has a little bit of continental crust "contaminating" some of the magma. It still remains that most of Iceland is not made of continental crust. $\endgroup$ – AlexP May 29 '19 at 22:35

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