True in general, and a good post, but one of the biggest problems with economics is the definition of utility :)
I think people DO obtain a utility from precious metals and diamonds -- otherwise they would never buy jewelry for such exorbitant prices and the markets for those materials would crash.
Coal also has a price based on the perception that another buyer will come along. This is true of all commodities. (One complication is if the commodity has an expiration date, its value may fluctuate with its age, but let's ignore that for now). There is certainly an options market for oil -- that's one of the reasons why its price fluctuates so wildly, and why certain airlines have lower prices than others when the price of oil is high (they try to purchase options when the price is low). For example, look at the "hedging fuel" section here:
http://en.wikipedia.org/wiki/Southwest_AirlinesAlthough Southwest is a company that actually uses the fuel that they purchase the options for, the presence of an options market shows that there are people willing to bet on whether the price of fuel will rise or fall in the future. Just like gold.
I'm not trying to say it's one thing or the other -- just that anything that has a value has a market for it, and anything with a value has a utility. The real question that you're trying to answer is, what is the liquidity of an item based on? Is gold our most liquid asset? This is a question that reflects its supply, demand, the ability to transport it, the rate of consumption... There have been many times and places when gold has not been a societies' most liquid asset (notable examples include salt in some historical African societies, seashells in others -- here's a neat wikipedia article:
http://en.wikipedia.org/wiki/Shell_money).
One difference between gold and putting your money in a bank is that gold withstands some of the effects of inflation. You can't print more gold. But you can mine more, and countries (Russia?) can try to sell their reserves on the open market and cause a drop in prices. I don't know very much about inflation so I'll stop talking now.