The phrase comes from The Emerald Tablet, or the Emerald Table as it is frequently called is part of the Hermetica and is one of most revered magical documents in Western occultism. Hermes Trismegistus is portrayed in art as holding an emerald upon which he inscribed the entire Egyptian philosophy.
http://www.themystica.com/mystica/artic ... above.html
Other sites attribute it to astrology and mysticism.
http://home.online.no/~luneng/999.html
http://www.sacredsexrituals.com/asabovess.html