So where did they get 'Devil With Seven Faces'?

OK, I understand that the producers and marketers tweaked the American title to make it sound more mysterious, etc. "DW7F" sounds a lot better than "A Bunch Of Italians Shoot Each Other Over A Jewel." But why that title? What's the rationalization?

There's no literal devil, and no one here has multiple personalities (for 'seven faces). Tbere are twin sisters, of course and ***spoiler alert** "Tony" turns out to be a creep. There are only two gangs after the diamond. And the diamond itself doesn't seem to have that many facets (that I could see).

Any ideas?