It's hipster-cool to proclaim that diamonds are bullshit, fed to us by The Man.
Cartels and market monopolies do exist. And mass marketing is a very effective tool.
People can try to mock the idea, but to denying those is pretty naive.
And as has been pointed out, there are ways to buy diamonds without supporting those cartels. This thread doesn't say that blood diamonds are bullshit, or that primary market (as opposed to second hand) diamonds re bullshit. And the comments, with a few exceptions, don't reflect those distinctions either. So sorry, no. This isn't about market monopolies or cartels or suffering mine workers or poor, dead Leonardo DiCaprio. It's trendeee to turn up one's nose at diamonds. You are correct that mass marketing is indeed an effective tool, and pop culture is second to none in that game.
Really, by the definitions used in this thread, all jewelry is bullshit in that it has no real function. Yet people who use alternative metals and stones somehow get a pass. Why is that? If this were really about people spending more than the functional value of an item should demand, or about buying into mass marketing, then a sapphire or a silver band would be equally mocked. The reason is that the mass marketing you mention has made it The Thing to be oh so above diamonds. You can try to layer on a veneer of social consciousness by claiming it is about working conditions and monopolies, but that is "bullsh*t", to quote this thread, because even diamonds that circumvent those issues are still scoffed at.
It makes people feel good to turn up their nose at diamonds and then go buy a sapphire ring, which probably supports many of the same companies that create issues in the diamond industry and which has the same bullshit perceived value that a diamond has.