I'm not sure which messages you're referring too. Diamonds have industrial uses, but they don't use gem-quality ones for that (and gold/silver have industrial uses too).

As a medium of exchange, gemstones are less convenient because they are less standard. The value of a diamond depends on its clarity, colour and cut as well as its weight, and you really need an expert to evaluate those. The value of gold - even gold jewellery - only depends on the weight of (pure) gold in it.