In the 1930s, De Beers, the South African diamond cartel, was in trouble. Americans weren't buying diamonds anymore, partly because of the economy, and partly because people found diamonds a bit tacky. So in 1938 the cartel hired an ad agency, N.W. Ayer, to strengthen the association in the public mind between diamonds and romance. Young men had to be brainwashed into thinking that diamonds were an indication of love; young women that diamonds were a crucial part of the courtship. And it worked. Between 1938 and 1941, De Beers' sales went up 55 percent.

The real challenge, though, was to make sure a secondary market for diamonds didn't arise, which would then depress world prices and make diamond investors very unhappy and not-quite-as-filthy-rich people. Luckily, a copywriter at N.W. Ayer came up with the slogan, "A Diamond is Forever," and that did the trick. The public—both in America and especially in countries like Germany and Japan—saw diamonds as "symbols of betrothal," to be kept for life rather than resold. (And good thing too; people who do try to resell their diamonds find that they're pretty much worthless on the secondary market.)

That—and a lot more—comes from Edward Jay Epstein's 1982 Atlantic piece, "Have You Ever Tried to Sell a Diamond?" The work of N.W. Ayer wasn't the only way De Beers grew so massive—the cartel's ability to manipulate the world supply of diamonds played just as big a part—but it's hard to find a better description of how powerful a deviously clever ad campaign can be.