I believe the black sword zoro recieved on thriller bark from ryuuma was made of seastone. After all, it was said it's greatest characteristic was it's hardness and seastone is supposed to be harder than diamond. Seastone is also black, right? Correct me if I am wrong, but I think it is not an unreasonable presumption.
A couple minor things: Without at least one seastone sword, how will he fight against logias? Also, I believe hawkeye's sword is seastone.
Also, @Festizio:
http://www.onemanga.com/One_Piece/484/13/
In this panel it looks like Shuusui touches Kuma's hand, if it were made out of seastone it would have nullified his Paw powers and sliced through his hand, since that doesnt happen we can reasonably assume that the sword is not made of seastone.
I'm not particularly onboard with notactuallytom's theory, but I have a counterpoint to this [Festizio's] specific argument. The sea, and thus we can infer seastone, affects all the different types of DF users differently. Kuma's a Paramecia user, so his power wouldn't be negated-if the Paramecia fruit users all react the same way to DF fruits, then Kuma would just get weaker as he's in contact with the stone. Just like how Luffy got super weak underwater in the Arlong arc, but still maintained his ability to stretch, even when fully submerged. Logia probably only "solidify" when in contact with the sea or seastone because they're the only ones that can become intangible in the first place. And Logia can still use their powers even after they've become tangible due to water effects (Crocodile was pretty soaked in his 2nd and 3rd fights vs. Luffy).
I just read a little bit ahead.
http://www.onemanga.com/One_Piece/485/07/
The hardness is way higher then steels.
I know that seastone is superhard, but I don't think Zorro has the ability to cut through this unknown metal.
I mean, there are probably hundreds of Df-users, I don't think a Shichibukai like Kuma is only dependent upon his fruit power.