Greenstone, or Pounamu (the Maori word for "to be dark green"), refers to several types of hard, durable and highly valued nephrite jade, bowenite and serpentine stone.
Pounamu plays a very important role in Maori culture and is worn by both men and women. It is considered a taonga (treasure) and is protected under the Treaty of Waitangi. It is found only in the rivers of the South Island of New Zealand, known in Maori as Te Wai Pounamu ("The [land of] Greenstone Water").
Pounamu taonga increase in mana (prestige) as they pass from one generation to another. The most prized taonga are those with known histories going back many generations. These are believed to have their own mana and were often given as gifts to seal important agreements.
Viggo Mortensen, Aragorn in the Lord of the Rings movie trilogy, took to wearing a hei-matau around his neck.