The length of a meter will be denominated in centimeters, the value of one #bitcoin will be denominated in sats. Though we can use gravel and dollars, just like some people use inches and feet…
Discussion
The size of the gravel is not uniform so you can not measure length with it. The same is true about fiat. Even though the unit is called 1 dollar the size of it is not fixed in time.
You’re right that the size of gravel isn’t standardized, so the comparison I made is flawed.
It makes no sense to compare a unit of measurement (dollar) to a (somewhat) finite asset (gravel).
Just like the imperial measurement system makes no sense compared to the metric system. Yet people use both systems, but when exact data is required (I.e. landing a space ship on the moon) we use the metric system.