A diamond is forever
Advertising slogan for De Beers' diamonds.
De Beers have used this as a slogan since 1948, and continue to do so (as of 2016). Here's the announcement of the campaign, from the New York Times' Advertising News and Notes column, August 1948:
"De Beers Consolidated Mines Ltd., owner of diamond workings in South Africa, plans a fall campaign in leading national magazines which will stress the engagement-ring tradition. Four-color ads will reproduce paintings by well-known artists and carry the slogan 'a diamond is forever.' N.W. Ayer & Sons, Inc., Philadelphia, is the agency."
The slogan began appearing in the US press soon after that date, as illustrated here, from the Ohio newspaper The Chronicle Telegram, June 1950.
The notion wasn't a new one. In Anita Loos' book Gentlemen Prefer Blondes, 1925, is the line:
"So I really think that American gentlemen are the best after all, because kissing your hand may make you feel very very good but a diamond and safire bracelet lasts forever."