Overview
Greywacke rock
Greywacke is one of the rock types that ‘fuses’ with another rock type to form nephrite – the most common form of pounamu. This ‘fusion’ takes place under pressure at high temperatures deep within the Earth’s crust.
Greywacke is found widely in New Zealand. It is largely made up of sandstone, and is dominated by the minerals quartz, feldspar, and mica.
Greywacke is a sedimentary rock (formed from mineral and organic deposits underwater), and has been changed by heat and pressure.
Dunite rock
Dunite is one of the rock types from which New Zealand nephrite, the most common form of pounamu, is derived. It is an igneous rock – formed within the earth from molten material.
Dunite occurs within oceanic crust – the part of the Earth’s crust under the ocean floor. It is made up of olivine and pyroxene, two minerals that are rich in iron and magnesium.
Semi-nephrite rock
Semi-nephrite is a low-grade form of nephrite. In other words, it is a rock that is on its way to becoming nephrite but is not quite there in terms of mineral structure.
Semi-nephrite is dominated by large tremolite mineral crystals, which occur within a mass of smaller crystals. These differing crystal sizes create a structural weakness that contrasts with high-grade nephrite, which is very strong.
Semi-nephrite is more common than nephrite but still relatively rare. High-grade nephrite is often found near semi-nephrite.
Nephrite rock
New Zealand nephrite is the most common type of pounamu. It forms when two contrasting rock types, such as greywacke and dunite, are fused under high heat-and-pressure conditions deep within the Earth’s crust.
New Zealand nephrite is largely made up of the mineral tremolite. In nephrite, the crystal structure of the tremolite is tight and interlinked, giving the rock immense strength – similar to steel. This toughness, along with its ability to be shaped, cut, and polished, meant that nephrite pounamu was favoured by Māori for tools and weapons.
The depth of nephrite pounamu’s green colour is determined by small amounts of iron and chromium.
Bowenite rock
Bowenite rock is considered to be more than 500 million years old, and is therefore the most ancient type of pounamu known in New Zealand. It is very rare, and is found mainly in Anita Bay and Poison Bay in Milford Sound.
Bowenite is softer than nephrite pounamu, and has a different mineral formation. It is mostly made up of the mineral antigorite.
Bowenite is an igneous rock (formed within the earth from molten material) and has been changed by heat and pressure.