People of past eras would carve rocks into the shape of coins, in a proof of work format, to facilitate trade.
Do you know which region this photo comes from?
Guess it's more of generational thing... Yapstones is just a tiny bit of - odd - random info I picked up when reading the Riply's Believe it or not clips in the Sunday comics when I was a kid back in the 60's...
Nice little article about the stones being 'the original bitcoin' https://www.sciencealert.com/the-original-bitcoin-still-exists-as-giant-stone-money-on-a-tiny-pacific-island. In this case the blockchain is the communal knowledge of each stones ownership history.
But that physical contrast masks the stunning shared feature of bitcoin and rai – both forms of currency depend upon a public, community ledger system, that provides transparency about transactions, as well as security, and all without needing a centralised bank structure.