Obviously you and I see that even that line of reasoning alone jacks up demand for an awful business, but not all people have the capacity to reason that abstractly about how their behaviour plays a part in a very big aggregate.
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My wife did get this after some discussion, but we got stuck with a different problem: we asked virtually every local jeweller to create something with a lab-created diamond (or even a different shiny rock entirely) and all of them -- to our astonishment -- refused.
They only work with mined diamonds from the suppliers they have long-standing contracts with and can ensure are as ethical as they come. I'm sure they have their reasons but that was very frustrating.