Which company used the phrase "A Diamond is Forever" as an advertising slogan?

De Beers Group is an international corporation that specialises in diamond exploration, diamond mining, diamond retail, diamond trading and industrial diamond manufacturing sectors.

De Beers successfully advertised diamonds to manipulate consumer demand. One of the most effective marketing strategies has been the marketing of diamonds as a symbol of love and commitment. A copywriter working for N. W. Ayer & Son, Frances Gerety (1916–1999), coined the famous advertising slogan, 'A Diamond is Forever', in 1947. In 2000, "Advertising Age" magazine named 'A Diamond is Forever' the best advertising slogan of the 20th century.