Never. A currency is something taxes are denominated in. And those currencies are necessarily controlled by the authority issuing the tax or the power to tax l

Never. A currency is something taxes are denominated in. And those currencies are necessarily controlled by the authority issuing the tax or the power to tax loses it effectiveness. (see Greece for details) Everything else is just a transferable asset - intangible or otherwise. It's why gold isn't a currency. You can't settle taxes with it. A currency is the modern equivalent of a tally stick. It's what an authority uses to command physical resources.