Yes, it doesn’t make sense when real world logic is applied but these are the rules of Oda’s universe
Oda in this chapter has Franky, A GODDAMN SCIENTIST, say that Ragnir’s ice being indestructible makes it similar to Diamond.
Yes yes, I know, in our real world you can break diamonds with a goddamn steel hammer and a good swing. Who cares
Remember that Sea stone is also said to be as hard as Diamond. Sea stone, the thing no character in the story, including Yonkos, has ever destroyed.
Imu, the top of the verse, couldn’t break “Diamond Hard Ice”. That’s that.