Crimson Longinus
Legend
No, that claim doesn't even make sense for eberron or something far beyond eberron Because diamonds can exist in a few states.
Creating a scenario where that volume of diamond dust is equal value as the first two (uncut gem grade/cut diamonds) by using small or flawed diamonds in bulk is if the labor is so unfathomably significant that the cost is able to equal or exceed cut gems of a certain common size.
- Uncut Gem grade diamond: These can be cut & increase the value
- Cut diamond: These can be used as a display of wealth in jewelry or similar
- Uncut diamonds too small or flawed to be useful as a gem: Not much to do with these other than grind them up or something
- Diamond dust: This can be made from literally any of the diamonds above and give the same result, but the discard from cutting & unusable diamonds are going to be the most cost efficienr.... which is why you can buy 3000carats of diamond dust for 30$ right now (~2268 carats make up a pound)
But now you're again making assumptions based on real world which might not be true in a fantasy world where these things are fuel for magic. For example in real world cutting diamonds make them more pretty, thus increase their value. But if spell fuel is the primary use of gems and the spell doesn't care about whether the gem is pretty, just about its volume, then by cutting the gem you're decreasing its value. If spells that require dust and smaller gems are more common and more useful than ones using larger gems and if cutting them to size (so you don't waste extra gem stuff) and powdering them takes expensive labour, such an equilibrium would exist, though as I said, it would be exceedingly unlikely. However, I'd argue that in practice no one can take all these variables into account, so any prices you might arrive for these things are more or less arbitrary, so you might as well go for the ease of use. It doesn't really make things less realistic than you completely flawed real world comparison would.