Month: December 2025

W is for Wacke

This is a slightly expanded version of my letter “W” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “W is for “Ward Beach Zoophycos”. 

W is for Wacke

Greywacke stone from Gemstone Beach – dry.
Greywacke stone from Gemstone Beach – wet.

This stone is a wacke – in fact, it is a Gemstone Beach greywacke (pronounced “grey-wacky”) photographed for me by Chrissy for my blog series on stones from that beach – it’s the first stone in this Post. Greywacke is the most common stone on NZ beaches, providing what often looks to be large areas of boring grey. I hadn’t collected any greywacke specimens myself as these usually bland grey sandstones are not very interesting to a tumble polisher. However, they are very interesting for other reasons. In 2003, Heather Nicholson submitted a doctoral thesis at the University of Auckland, at the age of 72!, with the title “The New Zealand Greywackes: A Study of Changing Geological Concepts to 1985”. She chose as the title for a talk to the Nelson Rock and Mineral Club, “Greywacke: The World’s Most Boring Rock… though it isn’t really because it’s got some fascinating stories to tell”.

https://www.mindat.org/min-49126.html
Heather Nicholson did a PhD on greywacke. https://teara.govt.nz/en/speech/9067/heather-nicholson
A Westland beach dominated by greywacke stones. https://www.fergusmurraysculpture.com/new-zealand/southern-alps-and-glaciers-9-pages/vii-gillespie-s-beach/

Wackes are poorly sorted sandstones, the tiny grains of sand in them being angular, not rounded (Mindat). These grains are mainly quartz, feldspar, pyroxene and mica. Textbooks often refer to greywacke as consisting of such grains “chaotically” embedded in silt and clay. 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”.

Some New Zealand greywacke is over 300 million years old. Rock material was eroded from the ancient super-continent Gondwana to be deposited by rivers 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 poorly sorted grain sizes can be found in greywacke (Wikipedia). This means greywacke is a “turbidite” (see photo below, left), forming via deposition from sediment laden currents that move very rapidly downslope through water.

From Heather Nicholson’s talk on greywacke. https://www.nelsonrmc.org/uploads/6/4/4/5/64457583/greywacke_v2_copy.pdf
https://teara.govt.nz/en/photograph/9065/greywacke-under-the-microscope
Specimens in Te Hikoi, Riverton Aparima Museum.
From Te Aro, The Encyclopeadia of NZ. https://teara.govt.nz/en/photograph/5121/greywacke-pebbles-with-quartz-veins
Letters of the alphabet, veined greywacke. Found on Pinterest.
Veined greywacke. Found on Pinterest.

There are interesting links between Germany, Scotland and New Zealand in relation to the concept of greywacke. “Greywacke” is a word of German origin, from “grauwacke”, meaning grey sandstone. Te Ara The Encyclopedia of New Zealand notes that “grauwacke” was first used in the 18th century to describe rocks in Germany’s Harz Mountains. Ernest Dieffenbach, a German scientist who travelled widely in New Zealand between 1839 and 1841, was the first person to apply the name to local NZ rocks. English geologists regarded greywacke as an “uncouth foreign term”, but it was adopted in Scotland. Archibald Geikie’s text-book of geology, published in 1903, gave descriptions of greywacke, and helped persuade New Zealanders that it was an appropriate term for their most widespread rock.

In the 1960s some geologists argued that the term greywacke was vague and imprecise. A subcommittee of the NZ Geological Society recommended that it be dropped, but this was widely ignored. The term greywacke is possibly used more widely in New Zealand than anywhere else in the world.

The next Post in this Series is “X is for Xcellent Xample of an Xtrusive Igneous Stone on Xmas Eve”. An Index for this Alphabetical Series is here.

A Review of the “Sandatlas” Website

I wrote this Review for the December 2025 edition of “StoneSpeak”, the Newsletter of the Southland Geological and Lapidary Club. The following is a slightly fuller version. The Southland Club’s website is here and its Facebook Page is here.

I was recently doing some research about breccia for a couple of blog posts I was writing (“TumbleStone Calendar 2026: November – Breccia” and “T is for a Type of Tiny Tectonic Breccia?”). By far the best piece I found on breccia was on a website called Sandatlas. The piece is well-written, comprehensive, nicely illustrated, is not shy about being a bit technical but is generally quite readable, and it covers a wide range of relevant issues. It is not a brief piece and it doesn’t oversimplify, but it was invaluable in helping me gain a detailed overview of breccia and its formation. The accompanying photographs and diagrams are outstanding. The rest of Sandatlas is similar.

Siim Sepp. https://sandatlas.org/about
Sandatlas Home Page. https://sandatlas.org
Sandatlas page on Breccia. https://sandatlas.org/breccia

Sandatlas is the project of Siim Sepp (above, left), an Estonian who describes himself as a software developer, author, geologist, and photographer. He started Sandatlas in 2012 as a website about sand. But what started with photographing sand samples from around the world grew into a broader guide to geology. The website’s home page now states: “Sandatlas is an educational website that aims to explain geological concepts in a clear and accessible way. The content is designed for students, educators, amateur geology enthusiasts, and anyone curious about the Earth’s materials and processes. The site features illustrated articles about rocks, minerals, sand types, and key geological terms – supported by original photographs to help readers better understand the subject matter.”

One of the great things about Sandatlas is Sepp’s high-resolution photos of rocks – he often provides a good range of examples of a rock type, and they tend to be handful size or just a bit bigger, the size you find on a beach or in a river. The majority of the photos featured on Sandatlas are from Sepp’s own rock and mineral collection, assembled from numerous geological expeditions to places including Estonia, the Nordic countries, the United States, Cyprus, Morocco, France, and Ireland.

Gabbro specimen. https://sandatlas.org/gabbro/
Another gabbro specimen. https://sandatlas.org/gabbro/
A further gabbro specimen. https://sandatlas.org/gabbro/

One of the main pages on Sandatlas is “Complete Guide to Rock Types: Igneous, Sedimentary, Metamorphic”. Forty-six igneous rocks are listed, with brief descriptions, and 30 of them have links to their own page with a more detailed description and lots of interesting photos. Thirty sedimentary rocks are listed (19 having their own page), and 27 metamorphic rocks (16 with their own page). I assume Sepp intends eventually to provide pages for all the listed rock types. In general, the coverage is selective but is nevertheless impressive.

Igneous rocks. https://sandatlas.org/types-of-rocks/
Sedimentary rocks. https://sandatlas.org/types-of-rocks/
Metamorphic progression. https://sandatlas.org/metamorphic-rocks/

Another main page is “Rock Forming Minerals”. This is a very long page and is much more technical than the rock types section. It’s more like what you would find in a geology textbook, but in my view it is well organised, well explained and well illustrated. If you want to gain an understanding of the chemistry and character of something like framework silicates, or the garnet group, or corundum, this is a great place to go to. How and where minerals form is covered along with the general traits of hand specimens. Field tips, weathering and environmental notes are usually provided. These help to make the technical information more useful to the likes of you and me.

There’s a lot 0r interesting stuff on Sandatlas, although it’s not always easy to find. At the end of the home page is a list of articles that explain key geological concepts and natural processes. This includes such intriguing topics as “How Geologists Identify Rocks”, “What is Opaque Color”, and “Do Rocks Really Fit Into Three Categories?”. At the end of each article is a folder icon with one or two names of Sections in the website – if you click on these Section names, you will be taken to lists of other articles that are otherwise hard to identify.

Sandatlas article. https://sandatlas.org/how-geologists-identify-rocks/
Sandatlas on the 3 categories of rocks. https://sandatlas.org/rock-classification-problems/
Two Category Names for lists of articles at end of Sandatlas on the 3 categories of rocks. https://sandatlas.org/rock-classification-problems/

“Rocks are not silent,” writes Sepp in the “About” page of Sandatlas. “They carry the memory of oceans long vanished, volcanoes that once shook the sky, continents that drifted across the planet. Learning to read them is a way of regaining closeness to the Earth. When we look at nature with understanding rather than indifference, we begin to value it – and we protect what we value.” He continues: “Sandatlas is a bridge between curiosity and knowledge, between observation and insight. It… exists for the broad community of curious minds – hikers who pause at a cliff face, photographers drawn to patterns in stone, collectors who turn over pebbles just to know what they are. It is here for anyone who suspects that the landscapes they walk through are more than just scenery.”

The next in this Series is “A Review of ‘New Zealand Geographic’ Magazine Online“. An Index to this Series is here.

 

A Review of Jocelyn Thornton’s (1985) “Gemstones”

I wrote this Review for the November 2025 edition of “StoneSpeak”, the Newsletter of the Southland Geological and Lapidary Club. The following is a slightly fuller version. I previously did a brief Post on the same publication in TumbleStone Blog in April 2016. The Southland Club’s website is here and its Facebook Page is here.

Page 4 of “StoneSpeak” Newsletter, November 2025.
Page 3 of “StoneSpeak” Newsletter, November 2025.
Page 2 of “StoneSpeak” Newsletter, November 2025.
Page 5 of “StoneSpeak” Newsletter, November 2025.

Jocelyn Thornton’s booklet “Gemstones” was first published in 1985 by Reed Methuen in the Mobil New Zealand Nature Series. It was produced to be used in the field – it has slightly thicker pages than usual, a spiral binding, and it’s of a size to fit in a large pocket. It’s a guide for stone collectors, polishers and cutters to the kind of material that can be found mainly on beaches in New Zealand. Each section has photos of stones on one side, and a numbered key and description of stones on the other. It’s an invaluable resource for stone fossickers and lapidarists, and there is no comparable book around. I managed to buy a second-hand copy about ten years ago but it is now rare, and we are fortunate that it has been made available online.

Booklet cover.
Online cover http://www.onnaturesedge.co.nz/downloads/nzgemstones.pdf.
Index to online copy of Thornton’s “Gemstones”.

Jocelyn Thornton was born in Wellington, initially working as a journalist after studying English literature at university (see Micromounters Hall of Fame). In the mid-1960s, she took up rock and mineral collecting as a hobby then returned to university in 1977 to study geology. For many years she has been honorary curator of the Alexander Mackay Geological Museum at Victoria University of Wellington (see photo below, left).

Jocelyn Thornton (right) at opening of Research School of Earth Sciences, Victroia University of Wellington. 1985. https://tapuaka.wgtn.ac.nz/nodes/view/8505
https://www.baltimoremineralsociety.org/uploads/1/6/6/3/16636366/joeclyn_thornton.pdf

As well as “Gemstones”, she has written “The Field Guide to New Zealand Geology: An Introduction to Rocks, Minerals and Fossils”, also published in 1985 but revised and up-dated a couple of times since. This is a substantial publication, covering the country’s geology within a framework of geological eras but also New Zealand regions. Its photographs are taken from the road or the beach, showing formations accessible to the average traveller. It is aimed at the general public and beginners in geology.

In 2009, Thornton wrote the entry on “Gemstones” for Te Ara, the Online NZ Encyclopedia. This entry has some excellent photos of different types of stones, such as “Silicified Wood and Plant Material”, “Chert, Flint and Jasper”, and “Greywacke Pebbles with Quartz Veins”. In 2004, Jocelyn Thornton received the NZ Order of Merit for services to Earth Sciences.

To get back to Thornton’s “Gemstones” booklet: It used to be available on her husband’s website, but that disappeared in 2022. Soon after, it was posted on the information site of Tuatapere, a town in Western Southland. The site is called “On Natures Edge – Tuatapere” and its “What’s Here” section has a brief page on nearby Gemstone Beach which includes a link that brings up a pdf file of the booklet.

Thornton’s “Introduction” to her booklet is very good, covering the topics of gemstones, NZ’s stones, polishing and other lapidary techniques, access issues, and equipment for beach collecting. She acknowledges assistance from various local rock and mineral clubs. There are two main sections in the booklet: 25 pages on specific types of stones, with photos, and 7 pages on stones from different areas/beaches, again with photos.

Garnet. Page 9 of Thornton’s “Gemstones”.
Petrified Wood. Page 21 of Thornton’s “Gemstones”.
More Jasper. Page 18 of Thornton’s “Gemstones”.

A wide range of stones are covered, from greenstone to quartz to jasper to petrified wood. Thornton’s comments on them refer to their characteristics and where they can be found. The regional coverage in the second section is patchy (for example, Northland is not included), sometimes quite general (one page is dedicated to the East Coast from Hawke Bay to North Canterbury) and sometimes very specific (Birdlings Flat, Slope Point and Orepuki have their own pages). If you are interested in the likes of the stones from Orepuki (Gemstone Beach), don’t just look at the page on that location – also check out the pages on the kinds of stones to be found there, such as Garnet, Jasper and Quartz.

East Coast. Page 31 of Thornton’s “Gemstones”.
Slope Point. Page 35 of Thornton’s “Gemstones”.
Orepuki (Gemstone Beach). Page 36 of Thornton’s “Gemstones”.

If you want to identify a stone and have only a vague idea of what it might be, Thornton’s “Gemstones” might be able to help you, if you slowly look through its pages. If you are visiting one of the beaches she covers, the booklet will alert you to the kinds of stones you will encounter. There is a lot of material in it and I often consult it. One of its limitations is the quality of the photographs. I often wish they were brighter or more detailed, but maybe that relates to the technology of the time and the need to keep expenses down. It also has no index, so if I’m looking for information on thulite, for instance, it’s difficult to work out if it’s in there – in fact, I don’t think it is. And that’s another limitation of the booklet, it doesn’t include all the significant stones to be found at different places. It’s selective in its coverage, maybe due to the stones that were available to Thornton to photograph. However, this doesn’t detract from its great usefulness to anyone interested in New Zealand’s stones. Like any good thing, you always wish for more.

The next Review in this Series is of the “Sandatlas” Website. An Index to this Series is here.

V is for Veined Vein?

This is a slightly expanded version of my letter “V” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “V is for The Chevron Shape of Trace Fossils”. 

V is for Veined Vein?

Veined Gemstone Beach find.
A veined vein?

This is a Gemstone Beach stone I photographed in April 2023 when I was collecting images for use in the Series “Gemstone Beach and its Stones: An Introduction for the Passing Motorist” – it is Stone W92 in this Post. The stone has an untidy white vein (of quartz or calcite maybe?) running through the middle. And it looks like this vein itself has a thin brighter-white vein running through it. However, maybe the thin vein is a denser or more intense thread of the same mineral in the wider vein. Or maybe some tiny bubbles of water and/or air are present in that thin line, making it brighter. It is often hard to know what is going on in the stones we find.

Veins are a common yet fascinating feature in stones. This can be illustrated by three other Gemstone Beach finds that feature in the “Gemstone Beach and its Stones: An Introduction for the Passing Motorist” Series (see Stones W87, W103, W110 in this Post). In the first below, there are veins of different colours, thicknesses and direction. In the second are irregular opaque and transparent quartz veins. The third has cross-cutting veins:

Another Gemstone Beach find, probably argillite.
Very interesting texture to this stone.
Cross-cutting veins.

Veins are infilled cracks. When rock fractures due to pressure and stress, 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 and calcite, 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, though veined veins are probably very rare. For a reasonably technical account of vein formation see Alex Strekeisen, with Wikipedia having a more easily read account. Note that, 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.

The next Post in this Series is “W is for Wacke”. An Index for this Alphabetical Series is here.

U is for U-Shaped Trace Fossil Found Near Ure (Waima) River Mouth

This is a slight expansion and rearrangement of my letter “U” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “U is for Unusual Variations of Trace Fossil Stones”. 

U is for U-Shaped Trace Fossil Found Near Ure (Waima) River Mouth

U-shaped trace fossil in limestone pebble from near Ure (Waima) River mouth.

I have struggled to find a stone starting with “U” so I’ve decided to go with one I found about four years ago just south of the Ure (Waima) River mouth (see Day 24 in this Post). With a little imagination, this stone has a U-shaped trace fossil in it.

Between Kaikoura and Ward on State Highway One, not long after crossing from Canterbury into Marlborough, you come to a bridge over a river marked as “Waima (Ure) River”. Marlborough Online states: “The Waima (literally meaning white water, due to the limestone in the river bed) or Ure River has its mouth a few kilometres south of Ward, and is the southernmost river fully within the current Marlborough District… For much of its length of about 30 kms, it flows through rugged limestone gorges… The European name Ure presumably derives from the Ure River in Yorkshire which also runs through limestone country.”

Location of Waima (Ure) River mouth, south of Ward. Area in red is where stone was found.
The beach just south of the Waima (Ure) River mouth.
A scattering of lighter-coloured limestone pebbles.

Along this coast, you can find many limestone pebbles that contain trace fossils – lines of dark-coloured burrows or trails through sediment that have been preserved in the light-coloured stones. I have collected a number of them with a wide range of trace shapes and sizes. Here are four more from the same beach:

Criss-crossing trace fossils in limestone.
A common pattern of trace fossils in this area.
Nearly a U-shape.
Trace fossil in limestone.

On page 64 of “A Photographic Guide to Fossils in NZ” by H. Campbell et al. (2013), it is noted that these trace fossils can be found in Muzzle Group geological strata in Marlborough, in fine-grained limestone, marl, mudstone and siltstone, laid down in deep water (over 150 metres), the traces due to burrowing activity by something like an echinoderm. An echinoderm is a marine invertebrate, like a starfish, sea urchin, or sea cucumber, that has a spiny skin and a five-part (or more) radial symmetry. Other marine animals leave traces as well.

From page 64 of “A Photographic Guide to Fossils in NZ” by H. Campbell et al. (2013).
Trace fossil in limestone in situ, on beach south of Waima (Ure) River mouth.
Kaikoura coast beach stones, near boundary of Canterbury and Marlborough.

These stones are too soft to tumble polish successfully but a short tumble in fine grit removes a thin outer layer, smooths the stone and clarifies the traces. It also often leaves a nice smooth waxy-feeling stone.

For more on this type of stone see “Selection of Tumbled Ward Beach Stones: Part 1 – Trace Fossils in Limestone”.

The next Post in this Series is “V is for Veined Vein?”. An Index for this Alphabetical Series is here.

T is for a Type of Tiny Tectonic Breccia?

This is an expanded and rearranged version of my letter “T” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “T is for Timaru Today”.

T is for a Type of Tiny Tectonic Breccia?

Vein of breccia.

I found this stone on Gemstone Beach, Southland, in April 2023. What fascinates me are the tiny fragments in the vein that runs nearly the full width of the stone. To me, it looks like a brecciation, a kind of type of breccia. The fragmentation has occurred under a kind of tectonic stress, the forces of weight and pressure, as large masses of rocks move and grind. The stress results in tiny cracks and openings and fragmentation. Every now and then I come across similar stones, sometimes the fragments being within a vein, sometimes within only part or even all of the stone.

Usually a breccia stone is defined as one that is made up of angular clasts (fragments) cemented together in a fine-grained matrix. A “monomictic” or “monolithic” breccia is one where the clasts are of a single rock type in contrast to a “polymictic” or “heterolithic” breccia where the clasts represent more than one rock type. Definitions of breccia usually refer to large-scale masses of rock, and they do not cover this form of brecciation WITHIN a small stone. Having looked through some of my finds, it seems to me there is a continuum from an “earthquake stone” (where there is an offset but no vein between the parts) to a vein (where the crack has been widened and filled) to a kind of tiny tectonic breccia (where fragments of the stone are in a vein, as with the stone above) to a brecciated stone (where a larger proportion, if not all, of a stone has been fragmented and the veins between have been infilled).

To illustrate this continuum: “Earthquake stones” show how tectonic stress can cause small fractures without brecciation – maybe more movement is required to create fragmentation. Two Gemstone Beach examples:

Gemstone Beach “earthquake” argillite. The stress has not been enough to create fragments within the stone.
Maybe the vein in this stone was caused by tectonic stress – after infill, tectonic stress cracked the stone/rock and pushed the right hand side upwards.

Another two Gemstone Beach finds, probably argillite, these having tiny breccia in veins:

Gemstone Beach stone.
Gemstone Beach stone.

Two stones with tiny breccia from tectonic stress, the first from Seadown Beach, Kakanui, the second being a piece of Slope Point petrified wood:

Seadown Beach, Kakanui, find.
Slope Point petrified wood.

A stone with widely dispersed tiny clasts and one with small brecciation in one quadrant, both from Gemstone Beach:

Gemstone Beach breccia.
Another Gemstone Beach find.

Next are two Gemstone Beach stones in which the brecciation has involved most or all of the stone. In the second, the whole stone has been brecciated. It appears to me not to be a cementation of fragments from other stones but is rather a stone that itself has been brecciated in situ:

Gemstone Beach breccia.
Gemstone Beach breccia.

All of these breccia (above) would be classified as monomictic or monolithic, the breccia being from one rock type, usually in fact from one mother rock. They can be contrasted to the following four polymictic or heterolithic breccia:

Slope Point polymictic breccia.
Gemstone Beach polymictic breccia.
Gemstone Beach polymictic breccia.
Gemstone Beach polymictic breccia.

The next Post in this Series is “U is for U-Shaped Trace Fossil Found Near Ure (Waima) River Mouth”. An Index for this Alphabetical Series is here.

TumbleStone Calendar 2026: December – Hydrogrossular Garnets

The Introduction to this Calendar can be found here. 

The final month in the 2026 TumbleStone Calendar features five hydrogrossular garnets. They were all found on the Te Waewae Bay coast in Southland, four on Gemstone Beach and one a few kilometres west at Papatotara. In general, a variety of stones are known as hydrogrossular, some being translucent, some opaque. Colours also vary, depending on what other minerals are present. Hydrogrossular garnets usually feel waxy and they shine brightly when wet on the beach. “Grossular” comes from the botanical name for the gooseberry, and gooseberry green is one of the classical colours of a more translucent type. Hydrogrossular garnets are not crystals but occur as rock masses. They are probably the most sought after stones on Gemstone Beach, often being made into pendants. Maori used to use them as hammer stones, as they are heavier and harder than most other stones. Hydrogrossular garnet is one of 13 minerals first described from New Zealand and accepted as valid by the International Mineralogical Association – New Zealander Colin Hutton analysed some Nelson stones in 1943 and noted their uniqueness. The Nelson and Southland hydrogrossular garnets once existed side by side, but movement associated with the Alpine Fault has now separated them by some 400 kilometres. For more, see “Hydrogrossular Garnet” and the various links listed at the end of it.

Calendar Cover, 2026.
December.
December’s Hydrogrossular Garnets.

Stone One (top row, left, in Calendar) was found on Gemstone Beach on 6 March 2025, a day when paragliders appeared over the beach: xx

Stone One, December.

The green in this stone is reminiscent of hydrogrossular’s typical translucent gooseberry green but there are other minerals present, providing the white and brown clouds.

The green colour of Stone Two (top row, right, in Calendar) is a great example of the classic gooseberry-green hue of a hydrogrossular, and is a deeper green than most specimens. I came across it on Papatotara Beach on 15 August 2025, the furthest west on Te Waewae Bay that I have found one:

Stone Two, December.

It is not as smooth as some specimens but it is bigger than most I have found. I count it among the best half dozen hydrogrossular garnets I have picked up.

Stone Three (bottom row, left, in Calendar) is another Gemstone Beach hydrogrossular garnet, found on 20 March 2025. I found it near the cliffs at the back of the beach – even dry, these stones stand out:

Stone Three, December.

I take photos of unpolished stones while they are wet so that their details show. Water tends to bead off the waxy surface of hydrogrossular garnets. The small wet patch at top of Stone Three, just to the right of centre, also reveals its waxy character.

On 25 August 2025, I found quite a number of nice hydrogrossular garnets on Gemstone Beach – this was in fact, at the time of writing, the most recent time I had fossicked there, though I hope to return in March 2026. On that day in August, I found both Stone Four and Stone Five. Stone Four (bottom row, centre, in Calendar) is an example of the attractive opaque tan coloured variety of hydrogrossular garnet, with some bright yellow along the left-hand side:

Stone Four, December.

Stone Five (bottom row, right, in Calendar) is similar to Stone Three, a white garnet though a little more tansparent and with some larger bright white clouds in it giving a sense of depth.

Stone Five, December.

It will soon be time to put up the 2027 Calendar!

TumbleStone Calendar 2026: November – Breccia

The Introduction to this Calendar can be found here. 

The month of November in the 2026 TumbleStone Calendar features four breccia stones. A breccia stone is made up of angular non-rounded fragments, sometimes from different sources, cemented together in a fine-grained matrix. The matrix may be a mineral like quartz or calcite, or a material like silt or clay (in a petrified state). As University of Auckland Geology notes, if the fragments (also known as “clasts”) are rounded, the stone is instead called a conglomerate. Water is the most common cause of such rounding. Sometimes in a breccia, the fragments come from a single rock or rocks of the same type – these are called “monomictic” (“mictic” meaning “mixed”). Where the fragments comes from two or more rocks types, the stone is called “polymictic”. One of the main ways that breccia forms is when material accumulated on steep hill slopes or at the foot of cliffs consolidates into new rock under pressure and heat. Some breccias are produced by the fragmentation of rocks during faulting or tectonic stress, and volcanic breccias comprise small blocks of lava in an ash matrix, the product of a volcanic eruption. Two less common formation processes are when hydrothermal fluid fractures a rock mass, and when a meteor fractures rock at the site of an impact. Sandatlas provides a good detailed account of breccia and its formation.

Calendar Cover, 2026.
November.
November’s four breccia stones.

Stone One (top row, left, in Calendar) is a monomictic breccia from Ward Beach, Marlborough, found on 2 April 2025:

Stone One, November.

It’s likely to be limestone, a common rock in this area – it looks like the rock has fragmented under tectonic pressure and then heated water flowing through it has left something like quartz behind to fill the gaps.

Stone Two (top row, right, in Calendar), found on Gemstone Beach on 15 March 2025, is very unusual as it has a brecciated vein:

Stone Two, November.

This monomistic breccia stone is very similar to the one featuring in the Post “T is for a Type of Tiny Tectonic Breccia?” (not yet published), a green-hued stone with a vein containing tiny fragments of the same stone. As I note there, “Definitions of breccia usually refer to large-scale masses of rock, and they do not cover this form of brecciation WITHIN a small stone.”

Another green breccia is Stone Three (bottom row, left, in Calendar), found on Gemstone Beach on 27 March 2025:

Stone Three, November.

The green matrix is probably the mineral epidote which is usually described as “pistachio green”. The light-coloured fragments or clasts all look like they might be a fine-grained mudstone, again making this a monomictic breccia.

Stone Four (bottom row, right, in Calendar), found on Slope Point on 10 March 2025, is polymictic, its clasts coming from different sources:

Stone Four, November.

The fragments don’t stand out as clearly as in Stone Three but careful examination shows a range of them in terms of size, colour and make-up. Particularly attractive are the lightest coloured ones with tiny spots in them. These spots might be crystals or even tiny fragments – if the latter, this would mean that some of Stone Four’s clasts are themselves originally part of a different breccia.

For December’s hydrogrossular garnets, see this Post. 

TumbleStone Calendar 2026: October – Kakanui Quartzites II

The Introduction to this Calendar can be found here. The Calendar features groups of beach stones I have collected recently, a different group each month. The stones are rough (un-polished) and usually wet, showing their colours and details. A small number of these 2026 Calendars remain for sale, with postage within New Zealand included, for $35 non-rural, $40 rural. Two Calendars to the same address cost $60 non-rural, $65 rural. Three Calendars to the same address cost $85 non-rural, $90 rural. Email me at john.tumblestone@gmail.com.

Calendar Cover, 2026.
October.
October’s Kakanui Quartzites.

The month of October features five quartzite stones from Seadown Beach, Kakanui, North Otago – another five featured previously in the month of May. Quartzite is a hard, metamorphic rock that forms when a quartz-rich sandstone is subjected to high heat and pressure over time. This process causes the quartz grains in the sandstone to fuse together, creating a dense, durable rock. One of the characteristics of most of these Kakanui quartzites is the tiny clear quartz crystals or fragments that become visible in close-up photos. Another common characteristic is the cloud-like distribution of colourful mineral throughout the stone, often iron oxide. Some of the diversity of these features can be seen here and in this Post.

Stone One (top row, left, in the Calendar) has a more subdued hue than many Seadown Beach’s quartzites, though it contains intriguing clouds of subtle colour giving a sense of depth:

Stone One, October.

Parts of the stone look like what you’d see peering through a powerful telescope into a galaxy in outer space. I found it on 23 March 2024.

Stone Two (top row, right, in the Calendar), was found on very productive fossick on 25 August 2025. I wrote in the Post about that fossick: “My favourite find today is a cloud quartzite containing different hues and some tiny black dots.” The stone is an interesting specimen for it contains four of the major Kakanui quartzite colours – grey, light yellow, orange, and red:

Stone Two, October.

The black dots are likely to be iron or manganese. The characteristic tiny clear quartz crystals or fragments are present, especially in the lighter coloured area. The appearance of clouds in a three-dimensional way is also there.

Stone Three (bottom row, left, in the Calendar) is among the darkest of the grey Kakanui quartzites I have found:

Stone Three, October.

It was collected from Seadown Beach on 27 August 2025.

The most red of the 10 Kakanui quartzites in the 2026 Calendar is Stone Four (bottom row, centre, in the Calendar):

Stone Four, October.

It would have been easy to mistake this stone for a red jasper, but the grey area would not have fitted that identification and it contains the tiny clear quartz pieces of the other quartzites from this area. As with Stone Two, Stone Four was found on very productive fossick on 25 August 2025.

Stone Five (bottom row, right, in the Calendar) is a rich yellow/orange, found on 30 March 2025:

Stone Five, October.

Yellow is one of the more common colours for Seadown Beach quartzites, though yellow quartzites can be found on beaches north of Kakanui through to Birdlings Flat. Stone Five has a more intense colour than most, though the appearance of “clouds” of mineral within it is still present.

For November’s breccia stones, see this Post.

S is for Scoria

This is an expanded version of my letter “S” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “S is for Slope Point Stone”. 

S is for Scoria

Large red scoria stone (wet), Kai Iwi Beach.

On my current morning walks on Kai Iwi Beach (near Whanganui) with dog Jasper, I am getting a more detailed understanding of the range of stones to be found here. They are quite widely scattered but that makes it easier to spot interesting ones. For more on this beach and its stones, see “The Iron-Stained Stones of Kai-Iwi Beach” and “K is for Kai Iwi Beach Stone”.

About three weeks ago I became intrigued with a type of stone I had previously just ignored, and I think it’s scoria. It’s like pumice but harder, more dense, and feels rougher. Most of these stones are dark red, some are grey, and they often have small black and white crystals in them. Despite their roughness and many tiny holes, I decided to collect a few and tumble polish them, just to see the results. Some are now undergoing a 220 grit tumble (see NOTE ON TUMBLING below).

I’ve found a range of colours of scoria on Kai Iwi Beach.
A light brown specimen, showing various “bits” in it.
Early morning, on the way down to Kai Iwi Beach with Jasper.
This is where I find the scoria, among the scattering of stones on Kai Iwi Beach.
Kai Iwi Beach, awaiting some rain.
Jasper on Kai Iwi Beach, patiently waiting while I pick up stones.

As Wikipedia informs me, scoria is a pyroclastic, highly vesicular, dark-coloured volcanic rock formed by ejection from a volcano as a molten blob and cooled in the air. It is typically dark in colour (brown, black or purplish-red), and often basaltic in composition. Scoria has a relatively low density as it is riddled with vesicles (tiny hollows from gas bubbles), but in contrast to pumice, scoria always has a specific gravity greater than one and sinks in water.

Coincidentally, my wife bought some scoria recently for the greenhouse floor – it’s brighter and more chunky than the water-smoothed specimens I find on Kai Iwi Beach. Commercial scoria often comes from the Auckland volcanic field which includes hills of this type of rock, and it is sometimes excavated for building materials and landscaping. See Geotrip to “Big King Scoria Cone”.

Commercial scoria in the greenhouse – to the right are some of my swan plants waiting to grow a bit more before being put outside for the monarch butterfies.
Commercial scoria. Rougher and chunky.

Another coincidence: I live on Rapanui Road, and on Rapa Nui/Easter Island, reddish scoria was used to carve the pukao (topknots) for the famous moai statues, and even for the main bodies of some moai – see “Constructing the Rapa Nui moai”.

Scoria top knots on Rapa Nui moai. http://www.researchgate.net/publication/320236368_Using_Structure_from_Motion_Mapping_to_Record_and_Analyze_Details_of_the_Colossal_Hats_Pukao_of_Monumental_Statues_on_Rapa_Nui_Easter_Island/figures?lo=1
Rapa Nui/Easter Island moai with red scoria pukao on its head https://en.wikipedia.org/wiki/Scoria. This is a close up of the moai at Ahu Tahai, restored with coral eyes by the American archaeologist William Mullo.

An interesting comment on my Post in the Facebook Group was made by Bruce Taylor, a local fossil hunter: “Some of the stones found here used to be moa, as in they are from fossilised bones. Rare and hard to spot. Exceptional shark teeth also with fossilised root that is now stone. Wood, seal, dolphin, penguin, all rare but found and in stone. Examples can be seen at the camp ground.”

NOTE ON TUMBLING: I tumbled some scoria stones for a few days in 220 silicon carbide grit. The resultant slurry contained a lot of fine fragments that had come off the stones. Many of the tumbled stones felt relatively smooth, certainly a lot smoother than beforehand. Five of the more interesting ones:

Four more:

However, close inspection showed that, as could have been anticipated, there were still a lot of tiny holes and further tumbling would not have resulted in a smooth polished product. Tiny bits of scoria loosened throughout the process would also contaminate later barrels and loads, an issue identified by John Taylor (Scotland) in a comment on my Facebook Group post. I have therefore decided not to proceed any further with them.

The next Post in this Series is “T is for a Type of Tiny Tectonic Breccia?“. An Index for this Alphabetical Series is here.