I always viewed them like Star Trek "credits". You have a single storage device, the tomestone, that carries the information about how many tomestones it is, for example although you have the one small tomestone it tells you it contains the information of 2000 stones.

Like others have said, sort of like a microSD card. That way you only ever need one, but it can hold up to 2000 tomes, initially it holds 500 but gets upgraded after a while due to technological developments. If you view all true currencies that way you done need carts of gil or sacks of GC Credits. Pseudo-currencies like Atma & Alexandrite are carried singly.