I find it good maybe pluton is not made out of seastone but it won‘t surprise me if it is .
and the thing about volcanic stones made me think about it and this panel came to my mind .
In the animr version its even more clear the vapor of melting something in the background.
The Kozuki could even have played a cruital part in building the pluton ,with there craftmanship .
The made stones that lasted over 800 years so why not make a weapon out of it to last for the same time.
another reason for pluton beeing made of seastone could be the seakings don‘t see it like with marineships.
that could mean poseidon could not counter pluton
if thats the case it would be the same way around ,so any ancient weapons can‘t be stronger than the other to balance things out