It came from the deep.

Around this time of the year we often get cuttlebones washed up on the beach, sometimes in their hundreds. Cuttlebone is a hard, brittle internal structure found in those free swimming marine molluscs commonly known as cuttlefish. Cuttlefish live for 1 to 2 years and and then they reproduce, just the once,  and die. This cuttlebone is around 10 inches in length.

Cuttlebones are chambered, gas-filled shells composed primarily of aragonite, a form of calcium carbonate. The cuttlefish can regulate the relative amounts of gas and liquid within the cuttlebone thereby adjusting its buoyancy and its position in the water column. 

In the past, cuttlebones were collected ground and used as a polishing powder, including in toothpaste, and as an antacid to aid the digestion. Today, you are most likely to come across a cuttlebone on the beach or in a Budgerigar cage where they are used to provide calcium to the bird's diet.