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I've got a home-brewed D&D setting. Diamonds, in D&D, are needed for resurrection magic, so access to diamonds is literally life and death for any country with clerics capable of casting such spells. The outcomes of wars will likely hinge on which side can control access to diamonds better.

So, where do I put the diamond mines, relative to ecosystem, plate tectonics, etc?

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    $\begingroup$ Beneath layer 16, most commonly between 5 and 12. Make sure your players have an iron or diamond pick or they won't drop. $\endgroup$ – Piomicron May 27 '19 at 12:50
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    $\begingroup$ Diamonds should be dropped by monsters of about mid-level challenge, so put them relatively far from cities where it id inacessible to noobs. Experienced players will be able to mine diamonds from those monster's drops. $\endgroup$ – Renan May 27 '19 at 13:07
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    $\begingroup$ I like the idea of prospecting for monsters instead of boring old placer deposits. $\endgroup$ – Starfish Prime May 27 '19 at 13:18
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Your setting is magical. Clerics are servants of gods. With that in mind:

Anywhere you please

The presence or absence of diamonds in your world isn’t limited by such mundane factors as where volcanoes exist or whether ancient carbon deposits lie buried beneath the surface. You can literally have the Goddess of Diamonds decide to plop a seam of diamonds anywhere. Perhaps a wizard accidentally blew himself up and scattered diamonds across the landscape, or Earth Elementals decided to prank the surface dwellers by putting huge diamond deposits under a loamy flood plain.

Point is: literally anywhere can be a suitable location for your diamond mines as you have so many more justifications available for why the diamonds are buried there. Use your imagination, remember why you said they were there, then build that into your world in an inversion of the usual Worldbuilding ‘my things are here because my world says so’ to ‘my things are here, so my world must be this’.

Goddess of diamonds? Drop a buried temple into the mine. Ancient Accident? The diamonds are cursed and the wizard’s ghost is still around. Earth Elemental Jokers? They keep playing tricks on the miners.

Or you can go the route of boring old plate tectonics if you really wish...

If you do, the answer is still ‘pretty much anywhere’, as diamonds can form and be transported to the surface anywhere there has ever been an upwelling of material from the mantle (specifically a rapid form of eruption that transports the freshly made diamond up before they melt again). These kinds of upwelling can happen damn near anywhere that the crust was once thick enough to support it, but then the diamonds can hang around while the geology around them is modified by... well, geology.

Tiny amounts of diamonds can also be made anywhere a load of carbon rich material got trapped in a subduction zone, or even where meteors have impacted. In those cases the diamonds will be very small, but if your world only cares about the worth of the diamond (who decides if the diamond is worth 10GP??) and uses some form of free-market economy then even small diamonds will rapidly become worth mining.

You get the point. Put the diamonds where it works for your campaign. Justify their presence there later.

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  • $\begingroup$ That is not, in fact, true. Diamonds can only form in specific conditions in the upper mantle, 150-200 km down, and it requires old, thick, and generally stable crust to form under, then rapid transport to the surface via a diatreme eruption; if the magma comes to surface as per a normal volcano, the diamonds are dissolved back into the magma and disappear. So no, not in a subglacial volcano, not at a subduction zone (source rock is too shallow). $\endgroup$ – Keith Morrison May 27 '19 at 14:45
  • $\begingroup$ @KeithMorrison: Diamond formation in sinking coal beds is rare, but happens, and diamonds have been observed being deposited by glacial activity, or even being washed up on shorelines or produced in meteoric impact craters. I stand by my answer of ‘Anywhere’. $\endgroup$ – Joe Bloggs May 27 '19 at 14:49
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    $\begingroup$ @KeithMorrison: Feel free to write an answer on the ways our planet produces diamonds in-the-main though. The OP might not want to rely on geological oddities and Gods to explain their super-rich diamond mines. $\endgroup$ – Joe Bloggs May 27 '19 at 14:54
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    $\begingroup$ @Keithmorrison: You’ve read from exactly the same articles I have, but seemingly skimmed over different parts to me. Between us we almost form an entire expert! $\endgroup$ – Joe Bloggs May 27 '19 at 15:08
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    $\begingroup$ @JoeBloggs, between the two of us, one of us is a geologist who's worked with diamond producers and explorers in Canada and only referenced that article because I didn't feel like typing the papers on my shelf. There's a reason why diamond exploration isn't going on in, say, Hawaii or Japan. $\endgroup$ – Keith Morrison May 27 '19 at 22:29
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Almost no-one knows.

In a D&D scenario, it is underworlders who know where the diamonds are: probably kobolds or svirfneblin or some similar creature that is willing to work hard mining for a living. The svir who mine diamonds know that if the source is found out, powerful interests will move in and take it over, at best enslaving the gnomes to mine more and at worse torturing them until they give up the location of the diamond vaults, then eating them. So the svirsneblin are extremely circumspect. They employ a series of middlemen such that none of these (non-svirsneblin!) individuals can betray them.

A good campaign might be an expedition to a fallen svirsneblin mine. The svir hire your campaigners to oust the monsters which have seized control of the mine, depicting the invaders as a bunch of bumbling umber hulks and owl bears who do not know the value of what they have. The adventurers will discover that other powerful interests from topside and underneath have realized the same thing, and are all converging on the mine to claim it for themselves.

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    $\begingroup$ This made me laugh. “Diamonds? Oh yeah, they come from the underdark. Where in the underdark? If I told you I’d have to kill you. Seriously. Why are you laughing? Us gnomes are tiny packages of death! Stop laughing!!” $\endgroup$ – Joe Bloggs May 27 '19 at 21:09

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