The value of anything can be propped up by speculation, what varies is the value which can be maintained without speculation (e.g., in cases where the bubble bursts).
I think this is the use of discriminating among (A) resources reasonably expected to decline in value, (B) resources expected to 'hold value' modulo speculation, (C) resources expected to have ongoing returns with or without speculation.
Glass also has decorative value. There's no easily imaginable risk of everyone suddenly deciding that it has NO decorative value.
So what? That's not the relevant risk. The relevant risk is that whatever you are using as a currency will DECREASE in value. Say, from the level of something highly valued as a medium of exchange accepted everywhere, to the level of a mere decoration. Or from a peak value where you bought in big during a speculative bubble, to a lower value where (yes) it's still worth something, but worth less than if you had kept it in treasury bonds or an index fund or a farm.
What makes gold "real" value and other exchangeable items not "real" value?
You had better stick to talking about exchange value, because once you get into nebulous discussion of "real" value it becomes immediately relevant that gold is nonproductive at best and prone to fluctuations and bubbles in reality.
If you define the discussion of value as being limited to immediate short term exchange, then it's no surprise that you end up concluding that the only things of "real" value are whatever gives you liquidity. That is a part of the story, after all Berkshire keeps $20-$30 billion liquid as a matter of policy.
But that doesn't tell you anything about future returns or opportunity costs, and it is not the whole story. The idea that gold's value is "real" because others think it is real (or better, have long thought that it is real) is just as viciously circular as saying that currency's value is real because others believe it is real.
No one proposes to use sheets of paper as a currency, let alone as an investment.
Treasury bonds have exchange value, not because they are paper, but because people will accept them, for the right they reliably represent; because the government is good for that debt. Gold, too, has exchange value NOT because of its chemical composition - only because people will accept it, and they accept it because they believe they will be able to trade it in the same way. If it loses half its value due to a bubble popping or the global supply increasing over time, it's no better than a currency which lost half its value due to government policy. There is always something around of value, and if the post-apocalyptic situation is bad enough then gold will be worth only a little at best, if you can even find someone selling whatever you need.
Land is an example of something which can produce, as is stock in a company which sells things that are always in demand. This was the contrast being made against the block of gold, not paper
Do companies really want to sift through video slideshows for, say, engineering jobs?