Same as it is for coal - how long it takes to burn it. Which is a lot quicker.
On the other hand, flags, religious symbols, and monuments don't have a ton of intrinsic value either, but humans are funny that way, with their social contrivances.
Industrial diamond use is not limited to burning (in fact burning isn't really an industrial use of diamond as far as I know, there's plenty of cheap ways to heat stuff). Diamonds (mostly synthetic) are used for cutting, piercing (drill bits), abrading[0], anvils in very high-pressure context and electronics heat sinks.
Most of the mined diamond production and the vast, vast majority of synthetic diamonds are not gemstone-grade and are used industrially.
[0] for all three mostly as diamond coating on tools, though there are monocrystalline diamond edges and blades, mostly for surgical use and microtomy: https://en.wikipedia.org/wiki/Diamond_knife
Unlike coal or religious symbols, diamonds aren't just useful for fuel. The properties of diamond (e.g. transparent, very hard) make it a useful material. More so if the price comes down.
Interesting. Caloric value as the way to compare value of things rather than fiat money. That's how it used to work right? Maybe we fall back to that after society falls?