Which famous 20th century author worked as an advertising copywriter and created the iconic slogan "A diamond is forever" for De Beers in 1948?

Throughout history, some of the most memorable advertising slogans have been crafted by professional writers and literary figures before they became famous. These campaigns not only sold products but also represented significant cultural touchpoints in literary and advertising history. This trivia question explores the fascinating intersection between literature and advertising, highlighting how wordcraft has shaped brand identities over the decades.

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Frances Gerety (not actually a famous author, but the real copywriter behind the slogan)
F. Scott Fitzgerald (who did work in advertising before literary fame, but didn't create this slogan)
Dorothy L. Sayers (the mystery writer who worked for S.H. Benson advertising agency but didn't write this slogan)
Salman Rushdie (who worked at Ogilvy & Mather but decades after this slogan was created)