Narwhals have a single, long tusk, which is an incisor tooth normally from the left side of the upper jaw. These tusks can be 3 metres (nearly 10 feet) in length and weigh up to 10 kg (22 lbs). Rarely, (about 1 in 500) a male will grow two tusks. In extremely rare cases female Narwhals may also grow a tusk. The Vikings believed Narwhal tusks had magical powers and bought and traded the tusks for more than their weight in gold.

Source: Norfolk Museums and Archaeology Services