Tusks are elephants incisive´s. There is a pulp inside, and thy have "open roots" why the tusks continue to grow throughout their lifetime, even if they get shortened. About half the tusk is visible, the rest of the unexposed tusk goes inside the cranium, why poachers always kills the elephants so they can remove all the Ivory.

The heaviest tusks ever known, belonged to the Colombian Mammoth (Mammuthus columbi), these weighed 226kg or 498lb.

From Elephant Glossary