"Diamonds Are a Girl's Best Friend" is a jazz song introduced by Carol Channing in the original Broadway production of Gentlemen Prefer Blondes (1949), with music by Jule Styne and lyrics by Leo Robin. American actress and singer Marilyn Monroe performed the song in the 1953 film Gentlemen Prefer Blondes. Monroe's character, Lorelei Lee, is followed on a transatlantic ocean liner by a detective hired by her fiancé's father, who wants assurance that she is not marrying purely for money.

Source: Wikipedia