He probably knows. Silvers Rayleigh implied he knew and offered to tell it to SH (or was he only baiting them for testing their reasons and resolve?)
It's probably the last peace of missing history or something like that, not some large diamond to be taken. Probably they need to find all the secrets leading to it and fix the things before revealing it to everyone. And time was not ripe yet for him to do it by himself and that's why Shanks decided to bet on next batch and support the possible candidates. We will see.