Diamond industry unites to match 20th century's best ad line

London

What makes a diamond valuable? History shows that a slick marketing team and a killer tagline help.

"Diamonds are Forever", voted the best slogan of the 20th century by Advertising Age, helped De Beers convince people for almost 70 years that it's worth parting with two months' salary for a gem. Yet the end of its monopoly a decade ago blunted its role as the industry's marketing edge.

Challenged by Michael Kors handbags, iPads and vintage Bordeaux, gem producers including De Beers, Rio Tinto Group and Russia's OAO Alrosa last month formed the Diamond Producers Association to ensure a new generation views the jewels as the ultimate luxury and expression of romance. At stake is the …