Metal is cold-cast into the hollowed-out bone of a cuttlefish. The cuttlebone is unique to the cuttlefish, a cephalopod, and contains layers of porous calcium carbonate.
The interior of the bone is soft enough to carve with your fingernail, and when the calcium detritus is blown away a layered terracing is exposed which determines the surface of the casting. In some pieces the resulting metal casting is cut and shaped into another form using the layers inherent in the bone as a guide.