Nobody needs a gem-quality diamond (and the industrial diamond market wasn't really manipulated). It's not like they were colluding to raise the price of insulin or wheat.
The fact that people were willing to accept the idea that they needed a diamond is as much the responsibility of the people who wanted to buy them, as it is the responsibility of those who marketed and sold them.
(Ignoring the fact that the demand even for lab grown ones is caused by such accounts...)
They were marketing diamonds to girls in school , shaping whole new generation to value diamonds. How can we expect children to defend themselves against such manipulation? I find this whole campaign amazing in its reach and abhorrent in its content.
 "N. W. Ayer outlined a subtle program that included arranging for lecturers to visit high schools across the country. 'All of these lectures revolve around the diamond engagement ring, and are reaching thousands of girls in their assemblies, classes and informal meetings in our leading educational institutions,'"