I think there is a material difference between buying a diamond and directly facilitating the activities of the drug cartels. In theory everyone buys things that are made by people in shitty conditions because there isn't much direct visibility on the front end as to what kind of nastiness happened elsewhere in the supply chain.

If you want that to change you have to make it illegal to do business with such folks not hope consumers fix it for you via the magic of the market.

It's unclear to me those shitty conditions (e.g. those of iphone manufacture) are net harmful to the poverty-stricken areas they affect - but I'm not sure that's not true of diamonds.

I think it's also more jarring that diamonds are otherwise useless symbols of status. At least iphones trickle down in some way (e.g. allowing the proliferation of older gen smartphones even in poor countries).