What is the narwhal's long tusk actually made from?

A narwhal's long tusk is an elongated tooth.

The tusk usually grows from the left upper jaw and can reach about 3 metres in length. It is a modified canine tooth with a hard outer layer and a living, innervated interior. Most narwhals with tusks are male, although some females also grow one, and rare individuals develop two.

The tusk's spiral pattern is produced as it grows. Researchers have proposed roles in display, social competition, and sensing changes in the surrounding water. Narwhals have millions of nerve endings in the tooth, so the tusk is not simply a dead, inert spike.

The narwhal lives in Arctic waters and feeds on animals such as Greenland halibut, squid, and shrimp. Its tusk is sometimes described as a horn, but that term is anatomically inaccurate: it is a tooth, not a structure made of horn or bone.