Month: October 2023

This Series of Posts provides an introduction to Gemstone Beach (Orepuki, Southland, New Zealand) and a basic guide to many of the stones commonly found on that beach. Previous Posts = Part One: Location and Carpark; Part Two: Main Features of the Beach; Part Three: Sources of Information & What is a Gemstone; Part Four: Brief Introduction to Geological Terms for Stones; Part Five: Translucent Stones; Part Six-A: Stones that are Predominantly White; Part Six-B: Stones with White Spots & Crystals; Part Six-C: Stones with Opaque White Veins & Bands.

The previous three Posts looked at seven types of white stones, or white parts of stones, that can be found on Gemstone Beach: 1 – White Milky Quartz Stones; 2 – White Hydrogrossular Garnets; 3 – White in Other Coloured Hydrogrossular Garnets; 4 – Stones with Opaque White Spots; 5 – Stones with Opaque White Crystals; 6 – Stones with Opaque White Veins; and 7 – Stones with Opaque White Bands. This Post looks at 8 – Stones with Opaque White “Patches”. There could be other types of white stones on Gemstone Beach but these are the ones I am currently aware of that are most likely to be found.

8 – STONES WITH OPAQUE WHITE “PATCHES“

This section contains a wide range of stones with opaque white material in them. Some have white patches in a conventional sense, others have a variety of white material and shapes. I have struggled to organise the stones in some sort of logical sequence or set of categories, so they are set out in a rather haphazard way. I also don’t always know the identity of a mineral or stone. What I hope this section will do is encourage you to look more closely at a stone you pick up on Gemstone Beach, to see how it is made up of a variety of colours, patterns and minerals.

Sometimes a stone looks like it has a little white in it but closer inspection suggests that the whole stone is likely to be white originally but then it has been “flooded” with a colourful mineral; or maybe the white is the matrix for fragments of a colourful stone (a breccia):

As mentioned in the previous Post, a “breccia” stone (like Stone W118) consists of small angular fragments (clasts) of one or more rocks cemented together in a fine-grained matrix. The small fragments come from some form of erosion and their angular shape means they have not “travelled” far, they have not had the chance to be smoothed. A similar type of stone with rounded smooth fragments is called a “conglomerate”. A number of the stones shown below are also likely to be breccia.

Stones W121 and W122 below are white stones with darker and/or colourful areas – the former at first glance looks as if some small areas of minerals are in it but closer inspection suggests it may be, like Stone W122, a breccia stone with opaque white quartz being the matrix:

The next two stones have black and white “patches” but the shapes and mix of the contrasting materials are very different – both have very interesting detail:

Another two breccia, both with white fragments (clasts) but one with a light green-coloured matrix, the other with a brown matrix:

Some of the stones I have found on Gemstone Beach have “hints” of white and closer inspection is needed to see their patterns more clearly:

Maybe Stones W127 and W129 (above) are breccia and Stone W128 is quartz “flooded” with dark red iron oxide. Stones W130 and W131 (below) have “wedges” of white material but their details are very different – Stone W131 has a very interesting margin along the edge of the white:

Below is a selection of five other stones with white “patches” in them, again illustrating a diversity of colours and patterns:

The different ways that black and white “patches” can occur are well illustrated by the following three stones – Stones W137 and W138 are breccia but with contrasting colours of fragments, while Stone W139 appears to be a white quartz stone with black mineral inclusions:

The next four stones have small white “patches” that are part of unusual structures. Stone W140 has an unusual light green colour (the matrix), probably caused by the mineral epidote, with small white fragments (quartz?) – it is an example of a breccia where the matrix makes up more of the stone than the fragments or clasts. Stone W141 could also be a breccia but with the epidote material being the fragments – I am not sure. Stone W142, most likely a quartzite, has been subjected to stress and veins filled in the resulting gaps. Stone W143 is a green stone with gorgeous small orbs in it, along with small patches of white and transparent quartz – I’ve always imagined such orbs are created in heat flows within a cooling rock, but I also know that sometimes they arise as crystals grow within such rock.

The next two stones both have what at first sight appears to be small lone bits of white. However, closer inspection reveals that Stone W144 is probably a quartz stone flooded by a dark red mineral (maybe iron oxide). And Stone W145 may be a quartz breccia(?) with a singular opaque white clast – it is tempting to call it a “xenolith” (a “foreign” stone in a stone of different material) but this term is usually used in reference to igneous rocks.

The final two stones with white “patches” are both green quartzites with small areas of opaque white material. This white material seems different from the white quartz seen in other stones, and may be a different mineral.

Both stones also have a number of thin veins, mainly transparent.

The next Posts in this Series (not yet all available) will look at green Gemstone Beach stones. The first one, just completed, is on green argillite stones. 

DISCLAIMER: Over the past six years I have visited Gemstone Beach more than 150 times, also walking further along the Te Waewae Bay coast. But I am not a geologist – I am an amateur beach stone collector and polisher. I have done some reading and research about different types of stones, but my knowledge is limited and open to correction. Some of the photos used in these Posts were taken at Gemstone Beach in April 2023; some come from my fossicking trips there over the past three or four years; and a few were taken after I looked through my “finds” in my stone shed at home. All the stones are “rough”, not polished. The coverage of stones is not comprehensive and is biased by my own interests and what catches my attention. The photos are not perfect, often having some reflections in them. But I hope these Posts prove useful to the “passing motorist” and others who find themselves on this interesting beach. 

Go to GEMSTONE BEACH AND ITS STONES: AN INTRODUCTION FOR THE PASSING MOTORIST – PART ONE, LOCATION AND CARPARK

Return to GEMSTONE BEACH

HOME – FOSSICKING BEACHES – BEACH STONES – TUMBLE-POLISHING

This Series of Posts provides an introduction to Gemstone Beach (Orepuki, Southland, New Zealand) and a basic guide to many of the stones commonly found on that beach. Previous Posts = Part One: Location and Carpark; Part Two: Main Features of the Beach; Part Three: Sources of Information & What is a Gemstone; Part Four: Brief Introduction to Geological Terms for Stones; Part Five: Translucent Stones; Part Six-A: Stones that are Predominantly White; Part Six-B: Stones with White Spots & Crystals.

The previous two Posts looked at five types of stones, or parts of stones, that can be found on Gemstone Beach that are white: 1 – White Milky Quartz Stones; 2 – White Hydrogrossular Garnets; 3 – White in Other Coloured Hydrogrossular Garnets; 4 – Stones with Opaque White Spots; and 5 – Stones with Opaque White Crystals. This Post looks at 6 – Stones with Opaque White Veins, and 7 – Stones with Opaque White Bands. The next Post looks at 8 – Stones with Opaque White “Patches”. There could be other types of white stones on Gemstone Beach but these are the ones I am currently aware of that are most likely to be found.

6 – STONES WITH OPAQUE WHITE VEINS

There are lots of stones on Gemstone Beach that have white veins in them, though they may not be noticeable at first. Among them are the very common dull grey stones, such as Stone W82 below. The first three photos of Stone W82 are of the dry stone, showing a typical light grey colour, while the second set of three photos is of the same stone when wet, which darkens its colour.

Stones W83 and W84 are two more white veined grey stones, photographed dry. Variation in the thickness of veins is apparent. The veins are often in fact layers of white material, running right across a stone.

The next white veined stone is darker in colour, with a number of white veins:

Sometimes a white vein in a grey stone can have a rusty hue because of the presence of iron oxide:

As I currently don’t have many photos of grey stones with white veins, I asked Chrissy Lampitt if she would send some to me. Chrissy lives at Papatotara, 20 kilometres along the coast of Te Waewae Bay from Gemstone Beach, and has a blog called Tumble and Polish. I first met her in March 2023. She kindly provided the photos of Stones W82, W83 and W84 above, the stones being found on her local beach. They are the same kind of white-veined grey stone as found on Gemstone Beach (and on many beaches and in many rivers in New Zealand).

The following Gemstone Beach grey stone with white veins seems to consist of finer grains and is smoother to the feel. Cross-cutting transparent veins are also present:

The next two stones are also fine-grained and smooth, and their white rings could be either veins or bands. A “vein” is an infilled crack. When rock comes under pressure and stress, it fractures and cracks of different sizes open up. This allows hydrothermal fluids, carrying dissolved minerals, to enter the gaps in the rock. Over time, the dissolved minerals precipitate out of the hot water. Crystals, such as quartz, grow within the cracks and fill them. Often, as the stresses on the rock change their orientation over time, veins will crosscut and even overlie each other (as with Stone W87 above). For useful information on this process, see Geology Fundamentals, Wikipedia, and for a more technical account see Alex Strekeisen.

In contrast to a vein, a “band” is a layer that is an integral part of the original stone, such as a layer of differently coloured sediment in a sedimentary stone. It can often be difficult to tell veins and bands from one another. For Stone W88 below, the thin white offset lines could be very thin layers of white material deposited in sediment, and Stone W89’s thick ring could be a lighter-coloured layer of sediment as well, though it might be a vein instead – it can be difficult to tell them apart.

Note that the offset rings in Stone W88 are a result of stresses on the rock after it was formed – such stones are sometimes known as “earthquake” or “faulted” or “faultline” stones (see the second part of this Post for an example).

Stones W82 to W85 (and maybe W86) are highly likely to be greywacke, a coarse-grained sedimentary rock (a sandstone). Such stones dominate at Gemstone Beach. The following six photos are of specimens of greywacke with white veins from elsewhere, the lower three showing how imaginative and creative arrangements of veined stones can make quite effective patterns:

There are other examples of the creative use of white-veined stones further below.

Phil, a member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” made the following comment after reading the first draft of this Post: “The majority of veins in greywacke are in fact zeolite which is a product of the depth of burial (ie: the zeolite metamorphic facies). Quartz veins are present in greywackes however, but they tend to be minor compared to zeolite.” Most of us, including me, tend to assume that all of greywacke’s white veins are quartz. Phil provided a photo showing zeolite and quartz veins in greywacke (see photo below, far left): “In the photo, the milky vein is quartz and the white vein is zeolite. The thousands of white spots are feldspars, showing the sediments for this greywacke had a volcanic origin.”

Phil also noted that you can determine whether a white vein is zeolite or quartz because of their relative hardnesses – you will be able to easily leave a scratch on zeolite with a knife, but not on quartz. Zeolites are a group of minerals with a crystalline structure made up of silicon, aluminum, and oxygen. They include minerals such as natrolite, stilbite and phillipsite.

Stones W87 to W89 above are highly likely to be argillite, a fine-grained sedimentary stone (a mudstone). More information on greywacke and argillite can be found at the end of this section, following Stone W113.

There are in fact quite a variety of different coloured stones with white veins of different thicknesses that can be found on Gemstone Beach. The following 12 stones represent a sample in order of increasing vein thickness. A wide range of different stones are involved, and often the veins themselves have unique characteristics. The first six stones all have a thin white vein:

For the next six stones, the white vein gets thicker:

Some of the veins can be complex and some stones have a number of veins, some of which are white and some of them transparent. These reflect how a rock can fragment in other than a clean and straightforward way:

Sometimes, white veins appear to be infills of irregularly shaped crevices, as in this rhyolite and this breccia (pronounced “breh-chee-uh”):

It is possible that the white area in the rhyolite (Stone W104) is part of its “flow-banding”, being a band of white coloured minerals laid down at the stone’s formation.

A “breccia” stone (like Stone W105) consists of small angular fragments of one or more rocks cemented together in a fine-grained matrix. The small fragments come from some form of erosion and their angular shape means they have not “travelled” far, they have not had the chance to be smoothed. A similar type of stone with rounded smooth fragments is called a “conglomerate”. See this Post for more information on Gemstone Beach breccia. Photos of other Gemstone Beach breccia stones, of a variety of colours and fragment sizes, are below:

Next are some interesting patterns and variations of white veins in five stones found on Gemstone Beach – the first two have end veins nearly eroded away, the second of which makes a heart shape; the third (Stone W108) has a wavy line; Stone W109 has a small fore-shortened wave; and the fifth one has crisscrossing white lines :

The last three white-veined stones are among the most beautiful I have found on Gemstone Beach or nearby. They are rare, so are unlikely to be found easily and in a short time. Many of the white veins in the stones above have been quartz – these ones may be a form of chalcedony, often called agate. In Stones W112 and W113, the veins are transparent but have an opaque white lining. All three stones are hematite jaspers.

It is amazing what close examination of wet stones reveals on Gemstone Beach!

The imaginative and creative use of a range of differently coloured stones with white veins can be seen in the photos below. This includes the construction of circles and spirals, and the arrangement of veined stones that look like letters of the alphabet – there’s even a book, “Alphabeach”, of alphabet letter stones.

On Greywacke and Argillite, Common Bearers of White Veins

Greywacke and argillite are both very common in New Zealand and are often associated with each other geologically. “Greywacke” is a word of German origin, from “grauwacke”, meaning grey sandstone. As Hamish Campbell put it, in an article in “New Zealand Geographic” in 1999: “It formed from sand, mud, gravel, and silt that was eroded off existing land and dumped by rivers into the sea, there to be compressed over tens of millions of years.” In fact, greywacke is the bedrock of not only New Zealand but also the whole Zealandia continent, as explained so well in the excellent YouTube video “Greywacke Bedrock – The Backbone of New Zealand”. Material was eroded from the ancient super-continent, Gondwana, to be deposited in the sea along its eastern coast, forming the basis for Zealandia and New Zealand. Undersea avalanches and strong turbidity currents meant a mix of grain sizes can be found in greywacke (Wikipedia). The rocks were deformed by a range of immense forces along the plate boundary that now runs the length of New Zealand, opening up room for mineral veins.

Both greywacke and argillite are sedimentary rocks, with greywacke being a sandstone and argillite being a mudstone. A sandstone has grains that are visible to the eye while the tiny grains making up a mudstone are not visible without magnification (see University of Auckland on grain size in sedimentary rocks). The grains in sandstones and mudstones, and the fragments in breccia, are called “clasts”, meaning fragments or particles broken off other rocks by physical weathering. According to the University of Auckland, greywacke is “a variety of argillaceous sandstone that is highly indurated and poorly sorted”. “Argillaceous” means containing a significant amount of very fine silt and/or clay – Hamish Campbell refers to it as a “muddy sandstone”. “Highly indurated” means hardened by heat, pressure or by the addition of a cementing ingredient like silt or clay, a bit more than with most sedimentary rocks. “Poorly sorted” means that its angular grains of quartz, feldspar and other minerals present have a range of sizes. An excellent summary of the characteristics of greywacke and its formation can be found in this slide show. The University of Auckland also notes that because greywacke has been subjected to significant amounts of tectonic movement over a long period of time, it is often extremely deformed, fractured, and veined with zeolite or quartz. Stones W82 to W86 above are good examples of greywacke with white veins.

In an excellent article on greywacke in “New Zealand Geographic” in 1999, Hamish Campbell points out how important the rock is to New Zealand. “Greywacke has itself been eroded to form the soft and much younger papa sandstones and mudstones of Taranaki, Wanganui, Manawatu and elsewhere. In the South Island, all the central mountains are greywacke, and the Canterbury Plains are just a sheet of greywacke rubble sluiced out of the Southern Alps. Most of Marlborough, Canterbury and Otago are greywacke. Even the schists of Otago are derived from greywacke—it has just been more deeply buried, and transformed into schist. And even that is not the end of the stuff. Under much of our vast continental shelf lies more greywacke. Out to the Chathams, south to the Campbell Plateau, it is probably all greywacke.” (Also, see this Post for an interesting explanation for why greywacke-derived soils in New Zealand are excellent for growing grapes.)

Argillite is a hardened mudstone, consisting of much smaller grains than greywacke and feeling much smoother. In New Zealand, greywacke and argillite often exist side by side in layered bands or strata. And, like greywacke, argillite has been deformed, fractured and veined because of significant amounts of tectonic movement over a long period of time (see University of Auckland). Stone W88 above is a good example of fractured argillite. Below left is a photo of pale grey greywacke rock lying above a band of dark grey argillite – in the greywacke there are a few, mostly thick veins of probably quartz, maybe calcite, while in the argillite, the white veins are thinner and more dispersed (from a geothermal website).

For more on argillite, see Wikipedia and Terraindex.

7 – STONES WITH OPAQUE WHITE BANDS

I commented above that bands are layers that are an integral part of the original stone, such as a layer of differently coloured sediment in a sedimentary stone. I was unsure whether the white parts of Stones W88, W89 and W104 were bands or veins. From my observations on Gemstone Beach, there are far fewer stones with white bands than with white veins. Here are three candidates – the first one (Stone W114) I am sure of, the other two I am not so sure – I don’t know enough about veins and their infills to have any certainty.

Stone W114 is called a “banded argillite”. This is a good clue that it’s white lines are bands rather than veins. Grey and brown are the most common colours of the bands but in this case some white is apparent. Many of the banded argillites I have seen on Gemstone Beach seem to differ from other argillites there – they seem to have been subjected to higher temperatures, appearing more fused or melted, with a “pottery glaze” look.

Other banded argillite stones from Gemstone Beach and nearby:

Stone W115 (below) appears to me to be banded. It is a type of quartz with a well-defined white layer through it.

The white line through Stone W116 seems to me upon careful scrutiny to perhaps be a concentration of the white material (a band) found scattered throughout the stone.

It can sometimes be difficult to decide whether a white line in a stone is a vein or band. But it is always worthwhile looking closely to try to see what is going on in a stone.

The next Post in this Series looks at Gemstone Beach stones with opaque white “patches”.

DISCLAIMER: Over the past six years I have visited Gemstone Beach more than 150 times, also walking further along the Te Waewae Bay coast. But I am not a geologist – I am an amateur beach stone collector and polisher. I have done some reading and research about different types of stones, but my knowledge is limited and open to correction. Some of the photos used in these Posts were taken at Gemstone Beach in April 2023; some come from my fossicking trips there over the past three or four years; and a few were taken after I looked through my “finds” in my stone shed at home. All the stones are “rough”, not polished. The coverage of stones is not comprehensive and is biased by my own interests and what catches my attention. The photos are not perfect, often having some reflections in them. But I hope these Posts prove useful to the “passing motorist” and others who find themselves on this interesting beach. 

Go to GEMSTONE BEACH AND ITS STONES: AN INTRODUCTION FOR THE PASSING MOTORIST – PART ONE, LOCATION AND CARPARK

Return to GEMSTONE BEACH

HOME – FOSSICKING BEACHES – BEACH STONES – TUMBLE-POLISHING