The narwhal tusk is actually the left front tooth of the male which projects up to 3 meters; the length is dependant on the whale and the age.
The tusk of the narwhal was sold by Vikings and other northern traders as the horn of the legendary unicorn which were considered to have magic powers. Reportedly, during the sixteenth century, Queen Elizabeth purchased a narwhal tusk for the cost of ten thousand pounds - the cost of a castle. The tusk was used as her scepter.
The tusk is believed to be a integral part of the mating process, as the males have often been seen crossing tusks during mating season, as well as using them during fighting for dominance and the right to mate with the female narwhals. Female narwhals have a shorter, and straighter tusk.
Even though the tusks grow with age, there is currently no reliable way of determining the age of a narwhal.