Has it ever been stated that One Piece is some kind of material treasure?
Oda himself confirmed in an interview with Momoko Sakura that the One Piece isn't something like &quot;the journey itself was the real treasure&quot; and that it is, in fact, a physical reward.
There is confirmation that the treasure will not be a spiritual gift.

Oda:&quot;Yes, I already decided on how it's going to end. Should I tell you now?&quot;
Sakura:&quot; .... Well, I want to know, but I probably shouldn't. BUT, the One Piece, it's not something like &quot;the growth your heart went through&quot;, right?&quot;
Oda:&quot;Hahaha, no no, I won't be pulling off some kind of Wizard of Oz thing like that. After having gone through an adventure like that, it wouldn't be fair to not actually give them a reward.&quot;

source: One Piece Wikia
This link contains details that are spoiler in nature. Please click at your own risk: Interview details + Spoilers