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

The narwhal's famous tusk is an elongated tooth.

Usually projecting from the upper left jaw of a male, the tusk is a long, helical canine tooth that can grow to around three metres. A small proportion of females also develop tusks, and some males have two. The tooth grows outward through the lip rather than forming a separate horn.

Narwhal tusks have unusual sensory properties. They contain millions of nerve endings and are exposed directly to seawater, which may help narwhals detect changes in their surroundings. Males also use tusks in tusking displays, in which they rub or cross them with one another.

The tusk is not essential for feeding in the way a spear might be. Narwhals mainly eat fish and invertebrates, especially beneath sea ice. Because it resembles a unicorn horn, the tusk helped inspire the medieval European unicorn legend.