Simplify the following text for a Grade 9 native English-speaking person.
Original: Ivory can be taken from dead animals â '' Russians dug up tusks from extinct mammoths â '' however most ivory came from elephants who were killed for their tusks .
Simplified: Ivory is a hard white material made from the tusks and teeth of animals such as elephants , hippopotamuses and walruses . Ivory is now very rare and expensive since there are restrictions on hunting elephants for ivory .