Greenstone, or Pounamu as the Maori call it, is a highly valued ornamental rock found on the South Island in New Zealand. Locations include Nelson, Westland, South Westland, Makarora, Wakatipu and Livingstone Mountains. In Fiordland, Pounamu is commonly observed around Milford Sound.
There are two ways to differentiate Pounamu types: Geologically or the Maori classification (appearance).
Geologically, the greenstone is divided in three types. Nephrite (jade), the most common type, is formed when greywacke fuses under high pressure and temperature in our earth's crust with another type of rock. Greywacke is a sedimentary rock consisting of sandstone. A contrasting fusion partner is dunite, a rock type made up of olivine and pyroxene. The silicate material tremolite or actinolite gives the nephrite it's stiff and strong structure due to an interlinked crystalline matrix. Nephrite is rich in calcium, magnesium and iron.
Its chemical formula is Ca2(Mg, Fe)5SiO22(OH)2. The higher the iron content, the greener the colour.
Bowenite is another type of greenstone and is mainly composed of the mineral antigorite belonging to the group of serpentinites. It is believed to be softer, rarer and older than nephrite. It's chemical formula is Mg3(OH)O4Si2O5.
Other serpentinites that are made up of one or more serpentine group minerals represent the third type of greenstone. A serpentinization is a process that transforms chemical and physical properties of ultramafic rock (rock that builds earth's mantle and is low in silica) through heat, pressure, oxidation and hydrolysis. In fact, these processes and formations are very complex but common at sea floors near tectonic plate boundaries. They are formed in rock until erosion settles them in riverbeds, the sea and along the coast.
The Pounamu was, and is, of high cultural value for the Maori which is reflected in many Maori legends. Early tribes such as the Ngai Tahu used the greenstone for tools, weapons, ornaments and even jewellery. All types are rarer than gold.