“Natural” money is opt-in money.

Bitcoin is opt-in money…

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“When a medium of exchange is generally accepted in society, it is called “money.” How does a commodity such as gold or silver turn into money? This happens through a gradual process, in the course of which more and more market participants, each for himself, decide to use gold and silver rather than other commodities in their indirect exchanges. Thus the historical selection of gold, silver, and copper was not made through some sort of a social contract or convention. Rather, it resulted from the spontaneous convergence of many individual choices, a convergence that was prompted through the objective physical characteristics of the precious metals.”

Excerpt From

The Ethics of Money Production

Jörg Guido Hülsmann