According to "Uncut", "The Marble Index" is "one of that rare breed of recordings which, the better part of four decades later, still has no adequate comparison, existing in a genre all its own". The album is considered a -goth record. André Escarameia felt the album "anticipated gothic rock by more than a decade due to ethereally darker [ambience and disturbing sonority." Its soundscape has been described as "bleak", "chilly", "harrowing", and "everything from the sound of someone rapping on a coffin lid to that of being buried alive". In her 1969 "Rolling Stone" review, Anne Marie Micklo described it as "mood music, with an obscure and elusive text recited over it". Regarding the record's sonority, British author Simon Goddard wrote, "it was on ["The Marble Index"] that the real sound of Nico was unleashed: a bleak pumping misery which would define her music for the last two decades of her life." Lenny Kaye of "Wondering Sound" described the album as "Circe-like".