I understand that, historically.  SOMETHING had to hold "value" to make trading easier.

Not to sound impractical, but what happens in the not-so-distant-future when we can mine for gold on asteroids? 

http://www.popsci.com/science/article/2011-09/meteorites-gave-us-all-our-extractable-gold-new-study-says

http://www.slate.com/articles/health_and_science/space_20/2014/10/asteroid_mining_and_space_law_who_gets_to_profit_from_outer_space_platinum.html