Also commonly referred to as “cuttlefish casting,” this simple, traditional metal casting technique takes advantage of the splendid properties of a finely porous bone-like internal organ that is shared by all members of the cuttlefish family. The animal uses it for buoyancy control. Though cuttlebone purchased commercially is likely to have been harvested from a cuttlefish caught for food, remnant cuttlebone from naturally-deceased animals can be gathered from beaches in many parts of the world.
The interior surface of the bone is composed of porous aragonite, a crystal form of calcium carbonate, that, in cuttlebone, is soft enough to take permanent impressions with moderate pressure. It is also naturally resistant to the heat of molten jewelry metals like silver and gold, though the mold is destroyed by the process of casting.
This tutorial from Jamie Hall of Ganoksin’s Primitive Method blog covers the process of making a simple cuttlebone casting, from a wax original, in sterling silver. It has good photographs and is clearly written and provides just the right level of detail for a beginner.