Despite their meteoric rise as speculative "assets," there are fundamental economic reasons why they will never act as a general medium of exchange despite the wild enthusiasm for them by the crypto-currency cultists.
Money – a general medium of exchange – is the most marketable (exchangeable) commodity in an economy. As a good, money is not sought after for its direct use – to satisfy individual wants – but to satisfy wants indirectly through exchange for other goods. Over time, one good becomes money since it possesses qualities superior to all other goods as a money. When gold became demanded not for its "use value," but for its "exchange value," it became a general medium of exchange – money.
As a consumer good, gold possessed a value or a "price" prior to it becoming a money, as the eminent monetary theorist Murray Rothbard explains:
...embedded in the demand for money is knowledge of the money-prices of the immediate past; in contrast to directly-used consumers' or producers' goods, money must have pre-existing prices on which to ground a demand.
But the only way this can happen is by beginning with a useful commodity under barter, and then adding demand for a medium to the previous demand for direct use (e.g., for ornaments in the case of gold.)*