Cuttlebone, also known as cuttlefish bone, is a hard, brittle internal structure (an internal shell) found in all members of the family Sepiidae, commonly known as cuttlefish, a family within the cephalopods.

Cuttlebone is composed primarily of aragonite. It is a chambered, gas-filled shell used for buoyancy control; its siphuncle is highly modified and is on the ventral side of the shell.^[2] The microscopic structure of cuttlebone consists of narrow layers connected by numerous upright pillars.

Depending on the species, cuttlebones implode at a depth of 200 to 600 metres (660 to 1,970 ft). Because of this limitation, most species of cuttlefish live on the seafloor in shallow water, usually on the continental shelf.^[3]

The largest cuttlebone belongs to Sepia apama

In the past, cuttlebones were ground up to make polishing powder, which was used by goldsmiths.^[4] The powder was also added to toothpaste,^[5] and was used as an antacid for medicinal purposes^[4] or as an absorbent. They were also used as an artistic carving medium during the 19th^[6][7] and 20th centuries.^{[8][9][10][11][12]}

Today, cuttlebones are commonly used as calcium-rich dietary supplements for caged birds, chinchillas, hermit crabs, reptiles, shrimp, and snails. These are not intended for human consumption. ^[13]

It can also be used in the process of pewter casting, as a mould.

Because cuttlebone is able to withstand high temperatures and is easily carved, it serves as mold-making material for small metal castings for the creation of jewelry and small sculptural objects.

Jewelers prepare cuttlebone for use as a mold by cutting it in half and rubbing the two sides together until they fit flush against one another.

Then the casting can be done by carving a design into the cuttlebone, adding the necessary sprue, melting the metal in a separate pouring crucible, and pouring the molten metal into the mold through the sprue. Finally, the sprue is sawed off and the finished piece is polished.

The act of using the cuttlebone specifically for mold casting is known as cuttleboning, and the practitioners as cuttleboners.

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