Elderling Creations Theory (Spoilers through all the books and Dragon Haven)

[Farseer]

Wizardwood is essentially made from memory stone, isn't it? The dragons swallow the black sand on the beaches of the Rain Wild River, then regurgitate it with their own saliva to create their cocoons. And we know that sand is simply the remains of stone that got carried to the beach from elsewhere by the wind and tides. So, the dragon's cocoons are made from the same black stone.

I concur!