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.
Mt Somers ‘diamonds’
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
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.
Ballast at Balaena Bay
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.