SOJs was a really interesting phenomenon and probably taught me more about economics more than anything else.
SOJs, "stone of Jordan" was a unique ring in diablo2.
Gold in diablo2 became worthless, even though they tried to soak it up with improving vendor items, and even implementing gambling.
Eventually SOJs became the currency for trading. You'd see postings like "2 socket archon 4 2 SOJ".
What was so interesting is that SOJ's are very similar to gold in the real world. They were easy to trade, easy to store (only taking up 1 square), very rare (or supposed to be), and had great intrinsic value because the sorcerer class was always running out of mana and they gave a 25% increase to mana.
Using SOJs as currency just developed naturally, probably just like gold did in the real world.