Gold, he learned, was considered to be a reliable store of value because extracting it from the ground required a certain amount of effort that tended to remain stable over time. When new, easy-to-mine gold deposits were found, or new mining technologies developed, the value of gold tended to fall. It didn't take a huge amount of acumen, then, to understand that the value of virtual gold in the game world could be made stable in a directly analogous way: namely, by forcing players to expend a certain amount of time and effort to extract a certain amount of virtual gold…
Economics of gold farming, Thanksgiving (prologue) - Reamde (2011) - Part I: Nine Dragons