Wano Theory

So, it’s been established that Seastone originated in Wano, and that it is an incredibly tough, nigh-unbreakable substance.

YET.

The fact remains that Seastone can come in a variety of shapes and forms, ranging from ship hulls, to handcuffs, to juttes, to nails. This means one of two things: Seastone can either be carved to particular shapes, or it can be molded to form particular shapes. And if it can be changed, it can be broken.

In either case, that means that Seastone is NOT indestructible; that there are means to physically change the substance, and since it nullify Devil Fruit users, that means it’s a process that either uses the byproducts of a DF power (I.e. Heat, etc), or it is a process that can be done without needing a DF to accomplish.

I predict that at some point in Wano, we'll learn of some weakness to Seastone, because honestly, if the Yonko were capable of being subdued by a mere Seastone handcuff, then the Marines probably would’ve tried already.