Latinum can't be replicated
Nobody ever says such a thing in Trek, though.
For all we know, latinum is just as easy to replicate as any other material. But it's processed, "pressed", just like paper is printed into bills. Bills have value because they are pressed with coded information that gives them value: thanks to their serial numbers, they serve as abstract promises of value.
Perhaps gold-pressed latinum is the most convenient medium for printing physical or chemical codes in the Ferengi world, just like a certain type of durable paper is in ours. Gold or latinum alone is merely pretty, and mainly useful for brooches and the like.
Timo Saloniemi