[parasubvert]

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.

[masklinn]

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

[SideburnsOfDoom]

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.