The background of the movie, "The Wizard of Oz".....you will find this very interesting.

"The Secret of Oz""The yellow brick, the emerald city of Oz, even Dorothy’s silver slippers (changed to ruby slippers for the movie version) were powerful symbols of author L. Frank Baum’s belief that the people – not the big banks -- should control the quantity of a nation’s money." 
Bill Still
The Secret of Oz
http://www.secretofoz.com/