It needs to be durable, transportable, divisible and difficult to counterfeit.
Famous example - cigarettes in a POW camp:
https://courses.cit.cornell.edu/econ4260/out/radford_pow.pdf
> The ownership of a large stone, which would be too difficult to move, was established by its history as recorded in oral tradition rather than by its location. Appending a transfer to the oral history of the stone thus effected a change of ownership
Compare with locations in a memory palace:
https://en.wikipedia.org/wiki/Method_of_loci
And to show that memory and consensus are what's important, it doesn't even matter if you lose the stone:
> In one instance, a large rai being transported by canoe and outrigger was accidentally dropped and sank to the sea floor. Although it was never seen again, everyone agreed that the rai must still be there, so it continued to be transacted as any other stone.