Her tail and fins - aka (most) of the black stuff is made out of a slightly transparent gel like goo substance, the denser it is, the darker it gets (No, that doesn't mean the fins are super tough, it just means they are way more solid, the fins are still thin. But it is possible to use the gel as a shield). The bones in her tail are magical gems / crystals / ...rocks that create and control the black gel, allowing the user to shapeshift the stuff into practically anything they want, and the more bones they have the more stuff they can do with the gel. The bones can be shattered rendering them useless, but she can eat bones to regenerate the bones (it takes a while though, and a fair amount of bones to be eaten beforehand).

The bones can be removed from her, and whoever has them can use them, but the amount of gel both her and others can shapeshift depends on how many bones they have and her tail / legs / other appendages can't be completely whole if a bone is removed and she doesn't shapeshift it to adjust for the lost section. If all of her bones (in her tail / legs) are removed then she won't have any legs / tail. She also can't make an infinite amount of bones (and by that i mean she can always generate bones but theres a max on how many she can have at once), i dont have a reason on why yet though lol, and also the bones themselves can't shapeshift.

- wips n close ups n stuff -