When it comes to genre defining moments, it is hard to think of too many more seminal ones than the riff to Iron Man. Played on the same scale that countless other tracks had used, but few with such great effect. Heavy as hell and so simple. Just perfect. Apparently when Tony Iommi and Geezer Butler first came up with the riff for Iron Man they called it "Iron Bloke". Seriously? Thankfully Ozzy Osbourne took the sound more literally and re-christened it before Geezer set to work finish the lyrics.