The Marble Index is the second studio album by the German musician Nico, released in November 1968 on Elektra Records. The avant-garde sound introduced in the album—a stark contrast with her folk pop debut, Chelsea Girl (1967)—was the result of the combination of Nico's droning harmonium and somber vocals, and the producer John Cale's musical arrangements, which were inspired by modern European classical music. Nico envisioned the release as an attempt to get artistic legitimacy and changing the looks that had made her famous as a fashion model.

Christa Päffgen, known by her stage name Nico, was a German singer, songwriter, actress, and model. She had roles in several films, including Federico Fellini's La Dolce Vita (1960) and Andy Warhol's Chelsea Girls (1966). Reviewer Richard Goldstein describes Nico as "half goddess, half icicle" and writes that her distinctive voice "sounds something like a cello getting up in the morning."