Month: May 2025

A recent Series featured 30 stones that I had sold to Doug Davidson, the journalist for Whanganui’s River City Press. Here is the article that came out of his interview with me. It is also now available on the newspaper’s website.

I was recently interviewed about stone collecting by Doug, a journalist for Whanganui’s River City Press. Afterwards he asked to buy some of my polished stones so I picked out 30 Gemstone Beach stones for him. When he took my photo for the article, Doug got me to hold a printout of the 30 stones.

Southland’s Gemstone Beach, located near the small town of Orepuki, has a great diversity of colourful and interesting stones. Their value lies NOT in being precious gemstones (because they are not!). But their value lies in the fascinating features and patterns within each stone, often only fully revealed in close-up photos. Tumble polishing the stones applies a permanent clarity to their surface so that their inherent visual art is always on display. My Post “March 2019 Stone Collecting Trip to Southern New Zealand – Gemstone Beach, from the Car Park to the Waimeamea River Mouth” describes the kilometre long stretch of beach that I walk in order to find these stones. I usually spend around three hours at a time there on each visit. In March 2019, in five visits, I collected about 18 kilograms of stones to take home to Whanganui to polish. These days, I pick up less stones, being much more selective, but I always find fascinating ones and often something new and unexpected (see the end of this Post, below).

To continue with Doug’s 30 stones…

Part One in this Series, covering Stones #1 to #5, is here. Part Two describes Stones #6 to #11; Part Three is about Stones #12 to #17; and Part Four features Stones #18 to #23. The final seven stones, the smallest, are now presented.

Stone #24 is a very strange stone and a mystery to me:

It looks like there is a lot of quartz in the stone but the top part of it has a significantly different appearance to the lower part. There is an interesting boundary band between the two as well. The top part has some different-hued patches in it but what catches the eye are the thin veins of a very dark mineral running across it. When I saw this on the beach, it immediately stood out as unusual. I wondered if it was a fossil of some kind, or a shell embedded in a stone. The photos show that it is all one stone, but with intriguing markings.

Stone #25 is the same type as Stone #9 (in Part Two), probably a quartzite with a fine lace-like tracery of mineral through it:

However, whereas the tracery is yellow in Stone #9, it is green in Stone #25. Patterns like these always catch my eye on the beach. The green is caused by epidote, a mineral we have seen before in Doug’s stones.

Stone #26 consists of small rock fragments (called “clasts” by geologists) from a variety of sources, making it “polymictic” in contrast to Stone #6 (in Part Two) and Stone #19 (in Part Four) which are “monomictic” (their clasts come from only one source):

However, is Stone #26 a breccia or a conglomerate? As I have previously noted, in a “breccia” stone, the clasts have not travelled far before being cemented together so they are angular and sharp. In a “conglomerate”, the clasts have undergone some rounding from the travel they have experienced, usually from water, prior to being cemented together. There is maybe some rounding of some of the fragments in Stone #26 but most fragments are angular, so it’s probably ok to call it a breccia. However, some stones are a mix, not fitting clearly into either category. It’s not unusual to find stones like that – the neat general categories of geologists are very useful but individual stones do not always line up precisely with them.

Stone #27 is a white quartzite:

Such quartzites appear to have thick “clouds” of intense mineral within them. There are some similarities to what I called a “canyoned” green quartzite I found on Gemstone Beach in March 2023. Another similar stone is the second one in this Post, a very dark green quartzite I found in March this year. On side B of Stone #27, you can see some holes that have not been smoothed out by the tumble polishing. They can ruin the look and the feel of a polished stone. However, to get rid of the holes in such a small stone would wear it away to practically nothing, and the stone is too interesting to lose.

Stone #28 is the kind of stone as Stone #21 (Part Four), a quartzite full of the green mineral epidote:

There is a prominent vein of translucent or transparent quartz running through side A of this stone. Such 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 can crosscut and even overlie each other.

I often discover that the smallest stones I pick up on Gemstone Beach are the most fascinating ones. When wet on the beach, they can catch my eye despite their small size. But their full beauty can’t be appreciated until close-up photos are viewed. Stone #29 is such a stone – it turns out to consist of a patchwork of colours and veins:

It’s a stone that has been subject to a lot of tectonic pressures and its veins have been flooded with different minerals at different times, contributing to its delightful patchwork.

Stone #30 is the smallest of Doug’s stones but to me it is the most gorgeous:

Like a lot of the stones I find and polish, it is mainly a mystery. I don’t know exactly what type of rock it is, though it will be one of the volcanics. It is amygdaloidal, like Stone #23 (Part Four) – a stone with many tiny vesicles (gas holes) that have been infilled with minerals. But I don’t know what these minerals are. When I picked up this stone from the beach and looked at it closely, I knew it would be interesting. But the photos made me gasp as they revealed a whole ‘nother world of tiny detailed beauty.

The rest of Doug’s 30 stones feature as follows: Stone #1 to #5 = Part One; Stones #6 to #11 = Part Two; Stones #12 to #17 = Part Three; Stones #18 to #23 = Part Four.

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Gemstone Beach has provided me with many fascinating discoveries – photos of six of my most valued are below. The first is a fossil coral, the only one I know to have been found on the south coast (and it’s likely to be very rare even for New Zealand). I glimpsed it on the beach in June 2020. Such a stone is the official State stone of Michigan where it is known as a petoskey stone. See “C is for (Fossil) Coral”. The second stone below is a bright pink thulite I found in September 2024. Thulite is the national gemstone of Norway where it was first discovered in 1820. I can sometimes find one or two thulites on a visit to Gemstone Beach, but not always, and the quality of their pink varies. More information on this mineral can be found in “January 2022, Stone of the Day #5 – A Little Pinky”.

The third stone below is probably chromium in diopside in hydrogrossular garnet, found in March 2024 (“chromium in diopside” is the intense green colour) – I used it for the month of January in the 2025 TumbleStone Calendar. I found the fourth stone, a poppy (orbicular) jasper, in September 2024. GeologyScience states: “The distinctive orbicular patterns form due to the presence of various mineral impurities or inclusions within the silica solution. These impurities can include minerals such as hematite, goethite, chlorite, or other oxides and hydroxides, which crystallize in concentric layers around a nucleus or central point, giving rise to the orb-like structures.” For a similar poppy jasper plus more information on orbicular jasper, see “O is for Opaque Orepuki Orbicular Jasper”.

The fifth stone below is a very rare white orbicular jasper I found in March 2025 – see “March 2025 Stone Collecting Trip – Part 25, A Small White Orbicular Jasper From Gemstone Beach, Friday 28 March”. The final stone below is a small jasper with chalcedony veins. It’s the size of my thumbnail. On the beach, it looked like a red jasper with some white patches plus some tiny veins. The veins seemed to have some bright white in them. The photos revealed some fascinating details – see “Southern Sojourn 2023(12): Surprises and Revelations, Gemstone Beach, Friday 10 February”.

I was recently interviewed about stone collecting by Doug, a journalist for Whanganui’s River City Press (here is the article). Afterwards he asked to buy some of my polished stones so I picked out 30 Gemstone Beach stones for him. In this Series of Posts, I present photos of each stone and provide comments on it. Part One in this Series is here.

Stone #18 has a deep orange colour, probably as a result of iron oxide:

The stone appears to have a lot of quartz in it. On especially side B, there are areas of opaque white quartz that contribute to its interesting character. Stones like these catch my eye on Gemstone Beach, and their high quartz content means that they will tumble polish well.

Stone #19 is a breccia, as was Stone #6:

The angular fragments in it indicate that they have not travelled far before being cemented together in the light-coloured quartz. Similar to Stone #6, it is likely to be “monomictic”, its fragments all coming from the one source. And it was probably a sedimentary rock originally. However, the fragments are not as densely packed as those in Stone #6, indicating they could have been floating in solution before being fixed in quartz.

Stone #20 is the third of the trace fossils in argillite, the other two being Stones #10 and #14. All three stones contain traces that belong to the ichnogenus Protovirgularia:

A trace fossil is technically called an “ichnofossil”. Such fossils are identified by their shapes, not by what made the shapes as this may not always be known. Furthermore, different animals could leave the same shaped trace. The term “ichnofossil” is derived from the Greek word “ichnites” meaning “footprint”. An “ichnogenus” is a group of trace fossils with similar characteristics. Ichnogenus Protovirgularia are trace fossil shapes consisting of “a small keel-like trail which is composed of an elevated median line and lateral wedge-shaped appendages alternating on both sides”, including lines of chevron shapes as seen in Stone #20. They were given this name in 1850 by Frederick McCoy, an influential Irish palaeontologist, who believed he was seeing the actual fossil of an ancestor of Virgularia mirabilis, the slender sea-pen, hence the term “proto-virgularia”. It was not until 1958 that the shape was identified instead as a trace fossil and one which was made by quite different animals. More information, with diagrams, can be found in my Post “I is for Ichnogenus Protovirgularia”.

Stone #21 is probably a quartzite full of the green mineral epidote:

Tiny bits of transparent quartz and white feldspar can be seen in this stone. But it has been flooded with an intense green mineral. Quite a range of green hues can be found in Gemstone Beach stones, and often the colour is due to the presence of epidote. Geologists often state that epidote is characteristically “pistachio green” – and Stone #6 comes close to that. The argillite of Stone #14 has also been coloured by epidote, the result being a much darker hue. However, epidote can also be responsible for the brighter more intense green as seen in Stone #21. For more on the colour green and the minerals that cause green in stones, see this Post.

Stone #22 is probably a small granite pebble, granite being made up of three minerals:

For geologists, a “mineral” is a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form (Wikipedia). Another definition is that a mineral is a naturally occurring, inorganic, solid, crystalline substance which has a fixed structure and a defined chemical composition (University of Auckland Geology). A mineral may occur as a large crystal, or as pieces between one millimetre to one centimetre in size, or as tiny microscopic grains. Minerals are the building blocks of “rocks”. Not many rocks are made up of only one mineral (one commonly-occurring example is quartz), with most rocks consisting of two or more. An often used example of a multi-mineral rock is granite, which is usually said to consist of three main minerals: feldspar, quartz, and mica. However, there are different types of feldspar and mica, and a number of variations of granite depending on the mix of minerals. The feldspar in Stone #22 is orthoclase, the pink/orange variety (see the comments on Stone #13) – this gives rise to what is called down south “pink granite”. If the feldspar is plagioclase, the resultant granite is predominantly white. In Stone #22, the orthoclase feldspar dominates even the quartz, giving it an orange hue. Mica is characterized by a layered, platy structure, consisting of thin, flexible sheets. It is common in a wide range of rocks, often appearing as tiny flakes. I have written about mica in “M is for Muscovite Mica from Joyce Bay”.

Stone #23 is known locally as a volcanic bubble-infilled stone, its more technical name being an amygdaloidal stone:

Molten magma often contains dissolved gas which can form bubbles in the rock as the pressure is released on eruption. These bubbles can get trapped in the solidified rock, forming tiny holes (called “vesicles”). These holes then fill with mineral-rich fluids which leave behind deposits of minerals such as quartz, chalcedony (agate), calcite and zeolites (zeolites are a group of minerals with a crystalline structure made up of silicon, aluminum, and oxygen). The resultant “spots” are called “amygdales” (also “amygdules”), a term that comes from the Latin and Greek words for almond, reflecting the almond-shape of many such in-filled vesicles. However, amygdales can also be round and some can be irregular shapes, as can be seen in Stone #23. Many of the dark-coloured stones with amygdales on Gemstone Beach are basalt.  

The next Post in this Series looks at the final seven of Doug’s 30.

Doug from Whanganui’s River City Press recently interviewed me about collecting stones for his article  “John Paterson’s hunt for coloured stones”. Afterwards he asked me if he could buy some of my polished stones so I picked out 30 Gemstone Beach stones for him. Part One in this Series introduced Southland’s Gemstone Beach and looked at the first five stones. Part Two featured the next six stones.

The first stone in today’s Post, Stone #12, is one of eight argillite stones in Doug’s 30, the others being Stones #3, #6, #7, #10, #14, #19 and #20:

Stone #12 is very fine grained and smooth. I used to think that it consisted of a dark layer and a light layer, with one of the layers eroding unevenly, revealing part of the layer beneath. However, I now think the two contrasting materials are mixed in together right through the stone, giving rise to its unusual camouflage-like or maybe turtle-back pattern. Argillite is a sedimentary stone. The three main types of rock are sedimentary, igneous and metamorphic. Sedimentary rocks are formed by the accumulation or deposition of mineral or organic particles on the surface of the Earth. They are thus formed from the weathering of pre-existing rocks and/or the pieces of once-living organisms such as shells. Layers of sediment accumulate and are then buried, compacted, and subjected to pressure and hot temperatures, up to 40 degrees Centigrade. You can often see a form of the original layers in the rock. Sedimentary rocks have not been heated enough to melt, and often can contain fossils (such as the trace fossils seen in Stones #10, #14 and #20). Most sedimentary rocks are classified according to the size of their grains or fragments. Siltstones and mudstones (like argillite) have grains that are not visible to the human eye and are very smooth. Sandstones have tiny grains that the human eye can usually distinguish and they sometimes feel slightly rough. Breccia and conglomerates have slightly larger fragments (see, for example, Stones #6 and #26).

Stone #13 is the type of stone I love to find on Gemstone Beach – it has tiny details of shape and colour that can only fully be appreciated in the close-up photos:

There’s lots of greys, oranges and whites in there, maybe feldspar and quartz, among other minerals. The dabs of pinky orange in Stone #13, a salmon pink to my eye, is a kind of feldspar. Feldspar is the most abundant mineral group in the Earth’s crust and contains a great variety of types. As Alex Strekeisen notes, there are more feldspars (60%) than all the other minerals combined in the outer crust. Plagioclase feldspar is one of the two main types. It is white and when in a rock it often looks like quartz. It weathers down to clay relatively easily. It is also commonly found in lunar rocks. Orthoclase feldspar is a potassium (alkali) feldspar which includes the orange/pink variety found in Stone #13, as well as in many granites.

Stone #14 is a trace fossil argillite stone:

My comments on Stone #10 provide some details about trace fossils and their argillite host rock. That stone, #10, is a grey/green colour, with the trace being thickish and an off-white colour. By contrast, Stone #14 is a much darker green and the trace on side A is more narrow, though longer, and an even darker colour. There are also traces on side B, less well-defined but of a similar colour. The two stones are a good illustration of the variety of trace fossil argillite stones. In July 2020, I produced a small photo-book which included a page (page 6) showing eight different specimens. While preparing the photos for Stone #14, I noticed some faint markings on both sides – these could also be trace fossils. Or they could be disturbances in the original sediments caused by water currents.

Stone #15 is one of those stones that becomes more fascinating the longer you look at it:

It is apparent that there’s a lot of little bits in the stone. The close-up photos reveal better detail of what’s there. There seems to be lots of tiny bits or crystals of quartz. Some of them could be transparent or translucent, some of them opaque. This relates to the three main ways of classifying how light passes through a stone. The first type is “transparent”, meaning light passes through the stone easily, as through clear glass or a clear quartz crystal; “translucent” means only limited light is able to pass through the stone, so that an object held behind it would look fuzzy; and the third category “opaque” means light does not pass through the stone at all (most stones are opaque). An interesting web page on the three categories provides the following useful illustrations:

Stones #16 and #17 are hydrogrossular garnets, Stone #16 being tumble-polished and Stone #17 being unpolished:

Gemstone Beach is especially well-known in connection with the hydrogrossular garnets that can be found there. As I note in a detailed Post on them, unlike the garnets we know as precious gems, hydrogrossular garnets come from dense rock masses, not crystals. Technically, hydrogrossular garnets are a calcium aluminium garnet with hydroxide partially replacing the silica found in other garnets. The first ever identification of hydrogrossular garnet in the world was in 1943 by Colin Hutton, from stones found in Nelson, and it is the most widely spread of the 13 minerals first described from New Zealand. More details can be found in “First Identified in New Zealand in 1943 – Revisiting the Hydrogrossular Garnets of Gemstone Beach, February 2023”. Hydrogrossular stones often feel waxy, and they have a slightly duller look, although when wet they shine brightly on the beach. Some are at least partly transparent. “Grossular” derives from the Latin word for gooseberry, referring to the light-green colour of some hydrogrossular garnets – Stone #17 is an excellent example. However, other minerals get mixed in, and white and brown are two other common colours of hydrogrossular stones. Hydrogrossular stones are easily polished and make excellent pendants.

The next Post in this Series looks at Stones #18 to #23 of Doug’s 30.

Doug is the journalist for Whanganui’s River City Press, a weekly community newspaper. He interviewed me about collecting stones (the article is titled “John Paterson’s hunt for coloured stones”), and afterwards asked to buy some of my polished stones. I picked out 30 Gemstone Beach stones for him. Part One in this Series introduced Southland’s Gemstone Beach and looked at the first five stones. The next six stones are featured in this Post.

Stone #6 is the same rock type as Stone #3, an argillite mudstone:

However, whereas Stone #3 is banded and a strong green colour, Stone #6 is brecciated (fragmented) and a pale grey/green. There are two different kinds of stones that consist of cemented together fragments, breccia and conglomerate. “Breccia” (pronounced “bretch-ee-ah”) comes from the Italian for “rubble”. In a “breccia” stone, the fragments that make up the stone have not travelled far before being cemented together and so they are angular and sharp. In a “conglomerate”, the fragments have undergone some rounding from the travel they have experienced, usually from water, prior to being cemented together. Stone #6 is a breccia because its fragments are angular. Three of the main types of breccia are named after the processes that caused them – tectonic, sedimentary and igneous. “Tectonic” breccia is where massive stresses on a rock tears it into fragments which are then re-cemented together in a fine-grained matrix. “Sedimentary” breccia arise when fragments from different stones have been briefly transported by water or submarine debris flows and laid down as sediment prior to becoming a new rock. “Igneous” breccia can be caused by volcanic eruptions and the accumulation of small bits of the erupted material. A breccia where the fragments represent more than one rock type is called “polymictic” while one where the fragments come from just one rock type is termed “monomictic”. Stone #6 is a monomictic tectonic breccia of argillite. In other words, the original argillite rock has been torn into fragments by pressure and movement while underground. For more about breccia, see University of Auckland Geology and “January 2022, Stone of the Day #2 – Breccia and the Giant Landslides of Fiordland“.

Stone #7 is another argillite, a different kind of banded argillite from Stone #3:

In Stone #3, thin dark green bands or veins are present in a green stone. In Stone #7, there are a number of thicker bands of grey, brown and black that make up the whole stone. In this type of banded argillite, it appears that some extra heat and pressure has been applied, as in places the bands kind of melt into one another. This is the most well-known variety of Gemstone Beach’s banded argillite, a similar one being in this Post.

Stone #8 is ignimbrite, a type of rhyolite:

Rhyolite is an igneous rock with a very high silica content. It is the volcanic equivalent of granite, granite bring formed underground. Depending on the nature of its emergence, rhyolite can also take the form of obsidian, pumice or ignimbrite. In certain situations, extremely porous (full of tiny holes) rhyolite lava flows occur which then collapse into a massy glassy material, obsidian, a dark coloured volcanic glass. If rhyolite is part of an explosive eruption, it can form the sponge-like pumice, a very lightweight, light-coloured rock full of tiny air holes – the only rock that floats. Ignimbrite forms from hot, dense mixtures of ash and pumice ejected from explosive volcanic eruptions. New Zealand geologist Patrick Marshall originally came up with the term “ignimbrite” in the 1930s meaning “rain of fiery rock dust” (from the Latin “igni” for fire and “imbri” for rain). The volcanic ash layers cool and compact under their own weight, becoming solid rock. I come across quite a few stones like Stone #8 along the south coast during my stone collecting trips and I love their banding and colour. They conjure up for me images of deep space and galaxies and stars, and the rings of planet Saturn. Many of them tumble-polish well, though their tiny gaseous pockets can sometimes interrupt a smooth surface. See the Post “November 2023 Polished Stones: Stone of the Day #22” which reports on how I came to identify such stones as ignimbrite.

Stone #9 is probably a quartzite with a fine lace-like tracery of yellow mineral through it:

When quartz rock is worn down to very small grains of sand, it can then be subject to heat and pressure. The grains become cemented tightly together and form quartzite. During that process, other minerals may enter the scene and provide a colour for the complete rock or leave lines as are seen in Stone #9. Some more information on quartzite can be found in the entry for Wednesday 26 May 2021 in the Post “FB Group Posts: 26, 27 & 28 May 2021 – Kakanui Quartzites and Slope Point & Gemstone Beach Stones”.

Stone #10 is one of three trace fossil stones in Doug’s 30, the others being #14 and #20:

I found Stone #10 on Gemstone Beach in September 2021 – it is the third stone in this Post. Trace fossils are not fossils of an animal but are fossils of the traces they leave behind. “Trace fossils are what is left of the activity of some ancient critter”, writes retired kiwi geologist Brian Ricketts in his blog “Geological Digressions”. That activity includes burrowing, moving, excreting, fighting and feeding. Stone #10 is likely to be an in-filled trail or burrow. The material making up the trace is slightly harder than the rest of the stone and is wearing away more slowly, causing it to sit higher. The stone is argillite, a hardened mudstone, the same as Stones #3 and #7. It was laid down in the Permian Era, 250 to 300 million years ago. I have long been fascinated by trace fossil stones. There are many to be found on Gemstone Beach, where they are known locally as “fossil worm cast” stones. I have collected a wide variety of them – see Stones Gn40 to Gn62 in “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Seven-A, Green Argillite Stones”. They are the stones I have done most research on and many Posts in my Blog deal with them – see for example “The Fossilised Worm Cast Stones of Gemstone Beach and Riverton – Part Five: ‘Burrowing Worms’ of the Permian in Brook Street Terrane Rocks East of Tihaka Beach”. Brian Ricketts provides an excellent general introduction to trace fossils here. 

Stone #11 looks similar to Stones #1 and #2 which are black igneous stones with white crystals:

However, the close-up photos reveal a much less fine-grained matrix. There’s a lot of white quartz in there, maybe some silvery mica, and some black mineral like maybe hornblende. My uncertainty is because it is often difficult to identify a stone. Geology textbooks and online sites usually refer to much larger rock strata and their characteristics in general, or to a sole specimen of a rock type. There is a great diversity of stones on Gemstone Beach, including a wide set of gradations within each type. I pick up stones because of their colours and patterns (like those with interesting black and white spots, as in Stone #11), and the likelihood they will tumble-polish well. They also have to be small enough to fit into my tumbler barrels. While I undertake research to try to identify many of my stones, it is not always easy or successful. So I don’t really know what kind of rock makes up Stone #11. I just know I like the look of it.

The next Post in this Series looks at Stones #12 to 17 of Doug’s 30.

Doug from Whanganui’s River City Press wanted to interview me about stone collecting for a local newspaper (see “John Paterson’s hunt for coloured stones”). So we chatted for an hour or so in a local cafe one morning. I brought along some of my polished stones to show him and later he asked to buy some.

I have picked out 30 Gemstone Beach stones for Doug. This Post features the first five, the largest, along with links to Posts which say something about them.  Part Two describes Stones #6 to #11; Part Three is about Stones #12 to #17; Part Four features Stones #18 to #23; and Part Five looks at Stones #24 to #30, the seven smallest. But first, Gemstone Beach is introduced.

Gemstone Beach is located just outside of the small Southland town of Orepuki, 70 kilometres west of the city of Invercargill. It lies towards the eastern end of Te Waewae Bay, a long sweeping bay of some 27 kilometres, part of Foveaux Strait between the South Island and Rakiura Stewart Island. There is a great diversity of colourful stones that can be seen on Gemstone Beach, many of them brought down to the coast from the Eastern Fiordland mountains by the Waiau River. A lot of the stones could be called “semi-precious gemstones”. “Precious gemstones” are ones like emeralds, rubies, sapphires and diamonds – you won’t find these at Gemstone Beach. “Semi-precious gemstones” are stones that look beautiful or interesting, that could be polished and made into a pendant, for example, but are not worth very much. I drive down to Southland from Whanganui two or three times each year to find stones to take home to tumble polish. This Post is a retrospect on my most recent trip.

The 30 polished Gemstone Beach stones I picked out for Doug (see photo above, far left) represent a glimpse into the diversity of stones to be found on the beach. In the photo, they are arranged from largest to smallest and are numbered. Stone #1 is 4.5 cms by 4 cms, Stone #30 is 2.5 cms by 2 cms. This Post features the first five of the 30, with the rest covered in later Posts.

Stones #1 and #2 (above) are black igneous stones with white crystals. Stone #1 is a “porphyry”, meaning the white crystals in it are relatively large, compared for instance with Stone #2. The white crystals are probably feldspar and the black is perhaps basalt. A number of similar black and white stones can be found on Gemstone Beach. Their crystals have formed as molten magma has cooled – atoms and molecules group together and “grow” into larger groupings, just as ice forms in freezing conditions. See The Conversation for an account of crystalisation in igneous rocks. The size of the crystals, their shape, and the ratio of white to black can all vary quite a bit. It depends on how quickly the molten rock has cooled and hardened, and what minerals are present to make up the crystals. There are two types of igneous rock, volcanic and plutonic. A “volcanic” rock cools on the earth’s surface and, because it has solidified quickly, the crystals have not had time to grow very large. But for a “plutonic” rock, the molten rock has cooled much more slowly underground, giving the crystals time to grow larger. A porphyry, like Stone #1, has well-formed crystals visible to the naked eye (called “phenocrysts”), set in a very fine grained matrix. For more, see the second half, called “5 – Stones with Opaque White Crystals”, of “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Six-B, Stones with White Spots & Crystals” (15 September 2023). Stone #2 is also a black igneous stone with white crystals in it but the crystals are a lot smaller. This means the stone cooled more quickly as it hardened, shortening the time available for crystals to grow. Stones like #1 and #2 tumble polish well because they are hard and the crystals are well embedded in them.

Stone #3 (above) is a green sedimentary stone, most likely argillite. There are eight argillite stones in Doug’s 30, the others being Stones #6, #7, #10, #12, #14, #19 and #20. In Stone #3, layers of green sediment of different intensities have been compressed together to create a form of banding in it. The “bands” could almost be called “veins”, made up of an intense green mineral. Argillite is a mudstone, consisting of much smaller grains than, for instance, greywacke sandstone. However, it has been hardened more than other sedimentary rocks, having been subject to a bit more heat and pressure. This means it tumble polishes better than most other sedimentary stones. Green argillite is very common on Gemstone Beach, and a red variety can also be found there. “Pakohe” is the Māori name for a grey argillite traditionally used to make tools, weapons, and other objects. It was valued for its hardness, strength, and ability to hold a sharp edge (The Prow). See “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Seven-A, Green Argillite Stones” for more – Stones Gn8 through to Gn13 are similar to Stone #3.

Stone #4 (above) is what I call a sugar quartz stone, and it is coloured by iron oxide. Quartz is a form of silica and is one of the most common minerals on Earth. Its purest variety is a clear colourless form known as rock crystal. A violet coloured variety is amethyst and a pink form is rose quartz – there are many more variations. The New Zealand geologist Jocelyn Thornton, in her booklet “Gemstones” (1985), notes on page 10: “Most quartz found will be massive, composed of small, interlocking crystals. Pebbles of this material are the most widespread pale stones on our beaches.” Wikipedia notes: “Milky quartz is the most common variety of crystalline quartz. The white color is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation.” For examples of milky quartz from Gemstone Beach, see Stones W1 to W3 in Section 1 of this Post. Stone #4 is not a milky quartz but is a more granular or sugar-like form of quartz. The iron oxide that colours it brown is a very common chemical compound in a wide diversity of rocks. This “rusty” compound of iron and oxygen often precipitates from heated iron-rich groundwater as it cools. Its presence can lead to yellows, oranges, reds, browns and blacks.

Stone #5 is a very dense and opaque form of milky quartz. It is uniformly white. It lacks the granular character of Stone #4, as well as its iron oxide staining. When found on the beach, few of these stones are totally smooth, often having a small chunk out of them or a deep scratch. Through tumble polishing, a smooth specimen, like Stone #5, can often be achieved.

The next Post in this Series features Stones #6 to #11 of Doug’s 30.