Who came up with the quote diamonds are forever?

DeBeers, the diamond company, came up with this advertising campaign in 1938.

Rumour has it that a secretary had worked all day on the words for the campaign, and just before leaving for the night, penned 'diamonds are forever' on her notepad. The next day, she presented the phrase to the campaign, and it became one of the most successful diamond advertising campaigns in history, wooing Americans to buy diamonds.