Of course a stock certificate has no intrinsic value - it's value is proportional to the ability of the issuer to redeem the certificate for actual livestock. Gold and other precious metals do have value in and of themselves; we value them for their appearance and physical properties, which are suitable to manufacture of jewelry and (today) certain electronic components, nanoparticles, etc.
As a currency, however, gold and silver have always required a state actor (i.e. "fiat") to guarantee metal content and enforce demand through taxation. This fact was made especially clear in the early Middle Ages. After the Roman Empire had collapsed in the West, gold coins (the solidus and triente) virtually disappeared from Europe, with silver denarii arising only as regional governments gained sufficient strength to enforce taxes. At first, these taxes would be paid "in kind" (essentially, food - the "in kind" produce of the land). Subsequently, taxes would be collected in coin, but only after the government had begun minting them and distributing them into the hands of citizenry.
Even during the "free minting" period, the value of currency vis-a-vis raw metal was determined via the mint fee and seniorage; free mints were free as in speech (if you had silver), rather than beer.
One important qualification is the low labor productivity at that time. The vulnerability to famine which obtains when the average worker produces just 1.8 person-years of food per year creates a high bar to any potential currency.
I highly recommend Peter Spufford's Money and its Use in Medieval Europe, which goes into insane-but-captivating detail on the above issues. It's an essential read if you want to understand the nature of pure fiat, metal-backed (convertible, less than 100% coverage) and metal-based (coins = weight of metal) currencies, and what features are shared by all three currency regimes.