[bcg1]

It doesn't have to be gold of course, but in absence of "royal fiat" as you put it... gold has uniquely superior natural properties to most other candidates (rare, fungible, can be divided into extremely small quantities without destroying it, does not tarnish, limited industrial use).

Silver has most of these qualities however it is second only to crude oil in its industrial usefulness (best conductor of heat, best conductor of electricity, shiniest metal when polished, antimicrobial/antibacterial, whatever you call its property that makes it suitable for photography, etc).

Of course you are correct about the use of spanish dollars... this was the origin of the US dollar, which was originally specified as certain quantity of silver in the Coinage Act of 1792. In colonial times it was not uncommon to cut a Spanish dollar into eighths (like a pizza pie) to make change... this is why even today we call $.25 "two bits" and why until a few years ago stock prices were quoted to 1/8 precision. Ironically most markets are electronic now and there are (coincidentally?) 8 "bits" to a byte to so maybe we never should have changed that at all ;)