"Why it matters where I got them from or why it matters whether they care?"

"First thing."

"Well, if it was just, say, a jewelry shop somewhere it wouldn't, I think, but that wouldn't make as much money as, say, larger diamonds, and those can attract lots of attention because they're rare enough to sometimes make the news and people would want to know that we didn't murder anyone to get one."

"Do mortals do that a lot?"

"Depends on what you mean by 'a lot'? There's like six and a half billion of us, in absolute numbers yes, but relatively speaking nnnot that much, and it also depends on where and who and why. The places with the most—precious minerals, I guess, tend to also not be very rich or well-developed places, often with corrupt governments, and correspondingly more likely to have crimes. And when we're talking about the kind of money some of those precious minerals can be worth," she shrugs.

"If a place has precious minerals and they're so expensive, why wouldn't it get rich selling them?"

"...the answer's very complicated and multidimensional, I think, and I don't have the knowledge to do it justice. But as a few examples, some places have more-or-less constant internal conflict for various cultural, ethnic, religious, or other historical reasons, so all the money made is used to feed said conflict rather than improve infrastructure stuff, education, health, what-have-you. Sometimes the money belongs to a very few, and the government's corrupt or de facto nonexistent and creating one from the ground up isn't easy especially when money is power and people with money don't want to be controlled by silly laws like 'don't kill people.'"

"Huh."

"It would be significantly better to have a different, less dubiously moral source of income like infinite energy or gate capitalism."

"But you're not murdering anyone for your diamonds."

"That's why I said dubiously moral not horribly immoral, I'm merely feeding a machine that works on murderously acquired resources."

"How does it feed the system to add non-murder diamonds to it?"

"By incentivising the sort of people who trade in murder diamonds."

"I don't see how it does that. You're not buying any murder diamonds."

"No, but I'm giving money to the people who buy murder diamonds via a channel that sells murder diamonds so they'll be a bit more likely to use this channel in the future, and not-me diamonds in that channel are more likely to be murder diamonds."

"What do you mean by a channel?"

"The disreputable sources who buy my non-murder diamonds don't want the diamonds themselves, they want to resell these diamonds, and they're also the sort of people who buy murder diamonds to resell."

"...but they don't have infinite money, so if they buy your non-murder diamonds they will not buy as many murder diamonds."

"But there are enough people in the world willing to buy diamonds from disreputable sources that they can be effectively considered infinite for these purposes."

"What, really? What do they want so many diamonds for?"

"It's not just about wanting many diamonds, it's more that we're lots of people. But also they're very pretty and are strong signals of wealth because they're rare, expensive, and absolutely useless."

"But they're not rare for you, you could make as many diamonds as you wanted until people stopped buying them!"

"But then I'd attract lots of attention. Even with disreputable sources, eventually someone'd realise the supply of diamonds had shifted beyond what's actually available on Earth, and people finding out about fairyland and sorcery is... in general inadvisable, without very careful management."

"What happens?"

"Well, typically when individuals do I think they just get lost forever here, but if society as a whole, or worse, individual groups of people did, the results are... much less predictable. And the worst-case scenario is pretty bad—a government with a military could take over fairyland and the Queen pretty trivially and I do not see this resulting in happiness and prosperity for mortals and fairies."