It is true that value can be stored in the form of gold or diamond for future use, however it again has certain drawbacks such as:
(i) Being limited in supply these were not available in abundance.
(ii) These involved certain transactions cost in exchange of goods for such metals.
(iii) These were not liquid form of exchange, because not everyone was able to accept it.
(iv) Divisibility was also a problem while dealing in these goods.