What is narwhal 'horn' really made of?

it is a horn grown on its forehead
it is an adapted scale
it is a canine tooth
it is an adapted nose bone
Usually, males have a single long tusk, the canine on the left side of the upper jaw that projects through the lip and forms a left-handed helix. The tusk grows throughout life reaching length of 3 m (10 ft). In some rare cases, both canines grow equally long, forming two tusks. In the Middle Ages, narwhal tusks were sold as unicorn horns.
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