I bet it's similar to how the Poneglyphs are unbreakable except for by the Kozuki, who knew the secret to chiselling the stone.
Both are things that someone (Joyboy) wanted to make sure weren't destroyed between the void century and now. Adam is probably a guaranteed refuge against the flood.
An indestructible rock. An indestructible wood.
Poneglyphs from the first. Noah from the latter.
Then, perhaps, the opposite too?
A message left in Adam wood, and a (black) ship of poneglyphs? (the Man Marked by Flames' ship?)
Saul just sitting on a wall like humpty dumpty for 20 years waiting for the Straw Hats to arrive so he can finally fall down and nearly die.
This is now my new headcanon. Saul was sitting on that wall, kicking his feet back and forth, waiting for Robin to return. And the moment he saw the SHs' ship, he became so excited that he lost his balance and fell to the ground, headfirst.