New Zealand lacks precious gems such as diamonds, emeralds and sapphires, but it does have gemstones that are prized by collectors and used for making jewellery. By definition a gemstone is beautiful, durable and rare. Stones of gem quality are free of flaws, have good colour and are large enough to be worked into jewellery.
Most beach stones have been washed down a river and tumbled in the surf, and are already round and smooth. Gemstones are also found inland, but it is almost always necessary to obtain permission from the landowner to collect these.
Finding gemstones requires knowledge of local geology, and patience. The best information on collecting stones in New Zealand comes from the various rock and mineral clubs, which hold regular meetings and exhibitions. Some clubs have negotiated permission to search for gemstones at localities which would otherwise be inaccessible. There is also a National Association of Rock and Mineral Clubs, which meets every year.
The main mountain ranges in both the North and South Islands are made of greywacke – a hard, grey muddy sandstone – and more than 95% of the river boulders that ultimately end up on the beaches are made of this rock type. Looking for gemstones is a matter of seeking out the small proportion of stones that are not greywacke.
Most stones look more interesting when they are wet or have been polished. Polishing entails the use of progressively finer grits to smooth the surface of the stone, with a polishing powder providing the final gloss. Most polishing of river and beach pebbles is done by tumbling: using a rotating or vibrating drum to polish many stones at a time. A cheap, but less satisfactory, alternative is to lightly spray smooth beach pebbles with a clear plastic coating.
The most prized and distinctive New Zealand gemstone is pounamu, which includes the mineral species nephrite and bowenite. Also known as greenstone or New Zealand jade, it is tough, workable, and beautiful when polished. Māori have traditionally made pounamu weapons, tools and ornaments, and used the stone for trading.
The main source of pounamu is the Arahura River, near Hokitika on the South Island’s West Coast. Pebbles of greenstone can sometimes be found on the beaches north of Hokitika, which is now a centre for pounamu craft work. Under the Ngāi Tahu (Pounamu Vesting) Act 1997, the ownership of all pounamu is administered by the Ngāi Tahu tribe.
Quartz is a major constituent of the earth’s crust as well as one of the most important rock-forming minerals. The chemical composition of quartz is silicon dioxide (SiO2) – also known as silica.
A distinctive feature of quartz is its hardness, which means that it polishes well and is not easily broken. The appearance of quartz is variable – it may be coloured or colourless; transparent, cloudy or opaque.
The many varieties of quartz can be divided into two main groups: crystalline quartz and very fine-grained (or cryptocrystalline) quartz, which includes chalcedony, agate, chert, flint and jasper.
Quartz crystals are hexagonal, usually shaped as a six-sided prism. Large, well-formed crystals of quartz are not common in New Zealand. They are mainly found in cavities of volcanic rocks in Canterbury and Coromandel.
Amethyst is a distinctive purple variety of crystal quartz. The colour comes from traces of iron affected by radiation, and some amethyst fades after long exposure to sunlight.
Quartz commonly forms veins in the greywacke boulders in rivers and beaches. Thicker quartz veins also occur in schist, which makes up the western edge of the Southern Alps and parts of Otago. When the rock breaks down, these veins become the source of the distinctive and widespread pebbles of white quartz that are found in Southland and most West Coast rivers south of Hokitika and on the nearby beaches.
No diamonds have been found in New Zealand, but every few years there are reports of diamond discoveries. There was excitement in the 1870s about the discovery of diamonds at Mt Somers, but these turned out to be quartz crystals – as have other reported diamond finds.
Quartz sands in some parts of Central Otago have been cemented by silica, forming a hard, yellowish-brown quartz sandstone that was used by early Māori for making flake tools. Blocks found on the surface or in rivers are locally called sarsen stones, and often polish up well.
Chalcedony (pronounced kal-sed-on-ee) is a waxy, translucent form of quartz with crystals so fine that they can only be seen under a microscope. It forms when water percolates in cracks and cavities – often formed by gas bubbles in volcanic rocks. Over time, the water deposits small amounts of silica in the cavities.
Coloured varieties of chalcedony are given different names. Carnelian – which is reddish-brown – is found at a number of localities around the Coromandel Peninsula. Agate is a type of chalcedony with parallel banding, probably caused by variation in the composition of the fluid that deposited the silica. Agates are prized for jewellery, and are found on Canterbury beaches south of Banks Peninsula (especially at Birdlings Flat) and around Coromandel Peninsula.
Opal is a softer form of silica that contains a small proportion of water. Small amounts are found on the Coromandel Peninsula, but so far precious varieties have not been found in New Zealand.
Chert, flint and jasper are opaque forms of silica that are usually formed as bands or nodules in sedimentary rocks. Chert is yellow to brown. Flint is a hard variety of chert, found as nodules in chalky limestone. It can be trimmed and shaped, and was occasionally used by Māori for making tools. Pebbles of chert and flint can be picked up on beaches near Kaikōura and along the eastern side of the North Island. Jasper is red or multicoloured, and commonly occurs with volcanic rocks.
Flint is common on southern English beaches and was often used as ballast for ships travelling to New Zealand. English flint pebbles are now found in several New Zealand harbours, where they were dumped when cargo was loaded and the stone was no longer needed to keep the ship stable. Balaena Bay in Wellington Harbour is a favourite place for rockhounds.
When wood or other plant material becomes petrified or silicified, the cell structure has been replaced by fine-grained varieties of quartz, mainly chalcedony or agate. The resulting hard rock is sometimes found as rounded beach pebbles. In the Coromandel area there has been considerable silicification of swamp material, which is known locally as Manaia stone. Cut slices sometimes show cross-sections of vegetation such as raupō stems and leaves.
Almandine garnet is an iron-rich variety that appears pink or red and is sometimes mistaken for ruby. It is found in many rocks on the West Coast. Larger almandine garnets are almost always too flawed for use in jewellery. Tiny crystals of almandine garnet are abundant in sand along the West Coast, often giving it a pinkish hue.
Calcium-rich garnet is called grossular. A red form, found in South Westland, is known as hessonite. Another variety, containing some water, is called hydrogrossular and was first identified at the Roding River near Nelson. It is also found on the beach near Orepuki in Southland. Rounded lumps of pale green hydrogrossular take a good polish and have been used for jewellery.
Hydrogrossular pebbles, being heavy and exceptionally hard, were used by Southland Māori as hammer stones for the making of stone implements.
Rhodonite, a pink manganese silicate, is found as lens-shaped deposits within schist in Otago and along the western side of the Southern Alps. It usually forms boulders with a rind of dense black oxidised material called pyrolusite.
Fine-grained mica and quartz accompany pink piedmontite in schist that is used as a decorative stone in Central Otago. When tumbled the mica grains tend to drop out of the schist, but it can produce a sparkling gemstone.
Goodletite is a local name for a rare and distinctive greenish-grey ruby rock – a form of the mineral corundum. This is found only near Hokitika on the West Coast. Despite more than 100 years of searching, goodletite has been found only as boulders and never in the place where it was formed. The composition of the rock suggests it comes from lens-shaped deposits of the mineral serpentine within schist, on the western side of the Southern Alps. Although rare, blue corundum has been found in Nelson and Southland.
Kyanite, an aluminium silicate, is one of the few blue minerals that occur naturally. Beautiful in jewellery, it is difficult to polish – and even harder to find. It occurs within schist, often in association with green chrome-rich mica, in a remote part of South Westland. Rare pebbles of blue kyanite schist can be found on the beaches between Jacobs River and Hunt Beach in South Westland.
Obsidian is dense, black volcanic glass that forms on the edge of rhyolite lava flows which have been suddenly cooled in contact with air or water. Good quality obsidian breaks with smooth, conchoidal (curved) surfaces, forming sharp blades that can be used for cutting. Obsidian is known from several localities, but the best material, much prized by Māori, comes from Mayor Island.
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Cooper, L., and R. Cooper. New Zealand gemstones. Wellington: A. H. & A. W. Reed, 1966.
Fernandez, Natalie. The New Zealand rockhound. Auckland: Boughtwood, 1981.
Thornton, Jocelyn. Gemstones. Auckland: Reed Methuen, 1985.