New Zealand – TumbleStone

What are Hydrogrossular Garnets?

I first came across reference to hydrogrossular garnets when gathering information online about the stones that could be found on Gemstone Beach, Orepuki, on the south coast of New Zealand’s South Island. Some brief descriptions of Gemstone Beach include comments along the following lines: “Semi-precious gems such as garnet, jasper, quartz and nephrite can often be found on the beach. A few hours beachcombing could easily yield gems such as hydrogrossular, jasper, fossil worm casts and the elusive sapphire” (quoted from information about the Riverton–Aparima South Coast Heritage Trail. See also Nature’s Edge: Tuatapere and the mindat.org entry on Gemstone Beach.) The source of this description could be a Heritage Trail sign that, to my knowledge, is no longer at Gemstone Beach (I don’t recall having seen it on any of my visits).

Gemstone Beach Heritage Trail — Source: https://talltales.me/2013/02/24/the-south-of-the-south/photo-2-01-13-10-30-21-am

The Encyclopedia of New Zealand mentions hydrogrossular garnets in the following terms:

Calcium-rich garnet is called grossular. A red form, found in South Westland, is known as hessonite. Another variety, containing some water, is called hydrogrossular and was first identified at the Roding River near Nelson. It is also found on the beach near Orepuki in Southland. Rounded lumps of pale green hydrogrossular take a good polish and have been used for jewellery. Hydrogrossular pebbles, being heavy and exceptionally hard, were used by Southland Māori as hammer stones for the making of stone implements

In many ways, all this information raised more questions for me than it answered. What does a hydrogrossular garnet look like? Is it a valuable gemstone, like other garnets? Why is it a garnet? How often can these stones be found on Gemstone Beach?

For a while, I mistakenly referred to them as “hydroglossular” (“…gloss…” not “…gross…”), thinking that because they were likely to be shiny they would be glossy. On my first few trips to Gemstone Beach, I decided that there was a particular stone that was probably a “hydroglossular”, even thought it seemed quite dull. It was a grey stone that was kind of a dull quartz-like thing, looking like there was water within its fabric (the “hydro” part).

For a brief time, I thought this sort of stone could be a hydrogrossular

I collected some and tried polishing them but they were very unremarkable stones.

So what does “hydrogrossular” mean? Wikipedia gives an answer that refers to the physical-chemical make-up of the stone:

Hydrogrossular is a calcium aluminium garnet series (formula: Ca3Al2(SiO4)3−x(OH)4x, with hydroxide (OH) partially replacing silica (SiO4)). The endmembers of the hydrogarnet family (grossular, hibschite, and katoite) depend on the degree of substitution (x):
grossular: x = 0
hibschite: 0.2 < x < 1.5
katoite: 1.5 < x < 3.
Hydrogrossular is a garnet variety in which a Si4+ is missing from a tetrahedral site. Charge balance is maintained by bonding a H+ to each of the four oxygens surrounding the vacant site.

So the “hydro” refers to “hydroxide”. Consulting Wikipedia again, hydroxide consists of an oxygen and hydrogen atom held together by a covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. (I will comment on other aspects of the above physical-chemical description in the next Post in this series.)

If you look up “grossular” in Wikipedia, you discover that “the name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia”.

It wasn’t until someone I met on Gemstone Beach about a year ago showed me some hydrogrossular stones he had just picked up, and actually gave me a couple, that I realised what they looked like and, just as importantly, what they felt like. They feel waxy, not the same kind of smoothness as a quartz stone, which is cool to the touch in contrast to the more “warm” feel of a hydrogrossular.

The Riverton Museum, “Te Hikoi”, has a small room which displays stones from the area, linked to an exercise of stone collecting set up for children on holiday. It includes a few hyrdrogrossular stones. With the permission of Museum staff, I took some photos of the display and of the drawers of rock samples in this room.

Riverton Museum, children’s rock collection display

Hydrogrossular garnet stones in Riverton Museum, children’s rock collection display

Riverton Museum, drawers of rock samples

In one of the drawers in the rock display room, Riverton Museum

Hydrogrossular garnet, Riverton Museum

The Southland Museum in Invercargill, now closed due to problems with Earthquake strengthening, had a Minerals display which included a non-smoothed rock of hydrogrossular garnet.

Hydrogrossular garnet, Minerals display, Southland Museum, Invercargill

Minerals display, Southland Museum, Invercargill

Southland Museum, Invercargill. Source: https://www.stuff.co.nz/national/102942792/southland-museum-and-art-gallery-building-closed-because-of-earthquake-risk

Tuatara statue, with petrified wood in background, sited outside the entrance to the Southland Museum, Invercargill

On page 34 of the excellent “A Photographic Guide to Rocks and Minerals of New Zealand” by Nick Mortimer, Hamish Campbell and Margaret Low (2011) is an entry on “Hydrogrossular”:

Five key points made in this entry are:

1) Its chemical composition is hydrous calcium aluminium silicate.

2) Hydrogrossular garnet hardly ever occurs as good crystals but rather as dense masses.

3) It can be found in the Nelson area as well as around Orepuki.

4) It is one of 13 minerals first described in New Zealand.

5) It was first identified by Colin Hutton in 1943.

In the next Post in this series, Why are Hydrogrossular Stones Called Garnets?, I will look at the first two points. Later Posts include Why are Hydrogrossular Garnets found in Nelson and Orepuki? and What is Hydrogrossular’s Place Among the Thirteen Minerals First Described From New Zealand? Also see Hydrogrossular Garnet on TumbleStoneTwo. Three significant later Posts on hydrogrossular garnets are February-March 2022 Fossicking Trip: Stone of the Day #15, Hydrogrossular Garnet from Gemstone Beach and February-March 2022 Fossicking Trip: Stone of the Day #19, Big Gemstone Beach Hydrogrossular Garnet and January 2022, Stone of the Day #20 – Brown Hydrogrossular Garnet from Gemstone Beach.

A Batch of Polished Stones from Riverton’s Back Beach

Panorama of Riverton’s Back Beach – Bluff Hill in the middle far distance, Stewart Island/Rakiura to the right, in the mist on the horizon, across Foveaux Strait

The 78 polished stones in a plastic storage container

These 78 stones were collected from the Back Beach in March 2019. They were initially tumbled in 320 grit as they were smooth beach pebbles. They were then tumbled in pre-polish and pro-polish tin oxide and burnished in borax. Altogether, these stones were processed in a 4lb rubber barrel for four weeks.

78 polished stones collected from Riverton’s Back Beach in March 2019, set out just having been dried from the opened barrel

Each stone has its own interest and attraction. The following are some of the most intriguing (Note: Some of the close-ups have been manipulated a bit to explore the patterns and colours):

The Back Beach at Riverton faces Stewart Island, some 40 kilometres away across Foveaux Strait, a rough and often treacherous stretch of water. The powerful waves are ideal for smoothing stones as they are tossed back and forth and up and down the beach.

Back Beach, Riverton, from the hill separating it from the Beach Past the Back Beach

On the Back Beach, looking back to the hill from which the previous photo was taken

The incoming wave dislodges stones, tossing them in the air and pushing them up and down and along the beach

The amount of stones on the beach varies considerably – in March 2019, a high bank of stones had been cut into in scallop shapes by the waves

The stones here are generally much smaller than the ones to be found near McCracken’s Rest, closer to the Waiau River mouth. Further up the beach, larger stones the size of your palm, can be found

Birds on the Back Beach stones

Visiting the Beach at McCracken’s Rest near Orepuki

I spent a few days based in Riverton early in May, on a stone collection trip. Two of my aims were to spend more time on Gemstone Beach and to explore the beach further to the west, near a place called McCracken’s Rest. I took an extra suitcase down with me so that I could carry more stones home with me than I usually do on the plane. I ended up bringing back 26.5 kgs of carefully selected beach pebbles.

Day One at Riverton saw me drive out to McCracken’s Rest, 36 kilometres from Riverton. This is a roadside lay-by and viewpoint eight kilometres west of Orepuki and Gemstone Beach.

McCracken’s Rest on the coast of Te Waewae Bay, northwest of Orepuki. The Waiau River mouth is further northwest, where the Te Waewae Lagoon is created by a stone bar. Source: Google Maps

Just northwest of McCracken’s Rest is the start of Te Waewae Lagoon.Source: Google Maps

McCracken’s Rest in relation to Riverton, 36 kms away. Source: Google Maps

On YouTube is this clip which gives a good sense of the roadside lay-by (although at 1:24 Stewart Island is misidentified – it is in fact well hidden in the mist – the piece of land referred to as Stewart Island is really the headland between Monkey Island and Cosy Nook, the headland just south of Orepuki – see the third last photo, bottom left, in the group below).

The information panel and viewpoint at McCracken’s Rest, looking out on Te Waewae Bay

The viewpoint lies between Orepuki and Tuatapere

On one side of the information panel is a map of the area

The photographer and the photograph

The Waiau River is the source of many of the stones on the beaches here

Te Waewae Bay

McCracken’s Rest and Gemstone Beach in relation to Monkey Island and Cosy Nook

The other side of the information panel features a historical time spiral for Tuatapere

The spiral of history

The beach between Orepuki and McCracken’s lies below cliffs all the way along so access is very difficult. At the viewpoint at McCracken’s Rest, I hopped over the fence and carefully made my way down the steep slope to the beach below.

The viewpoint at McCracken’s Rest – the best entry point to the beach, though unofficial

On the way down to the beach, layers of stones are exposed, their roundness suggesting they were probably laid down by the Waiau River or some other river long ago

Detail of exposed stones

The scene at the bottom of the slope, beneath McCracken’s Rest viewpoint, looking north-west

The beach at McCracken’s Rest is similar to the beach further south-east, back towards Gemstone Bach and the Waimeamea Lagoon. There is a low bank of stones above the high tide mark, along with a wide scattering of drift wood. Closer to the waves, there are sandy patches and drifts of smaller stones.

Looking north-west, towards Fiordland

Looking south-east, towards Gemstone Beach which is about 6 to 7 kilometres away

Sand and stones on the beach near McCracken’s Rest

There is no scarcity of stones

There are slightly more larger stones here than further south-east on the beach, back near Gemstone Beach

Stones on the beach near McCracken’s Rest

Looking back up to the McCracken’s Rest viewpoint

I spent two and a half hours there – the day was largely fine and with little wind, which allowed the sandflies to be active. I slowly walked (and fossicked) just over a kilometre north-westwards to the start of the Te Waewae Lagoon (created by the Waiau River trying to find a path to the sea). The actual mouth of the Waiau River can vary in position along this gravel bar, depending on the countervailing forces of the river’s flow and the stones thrown up by the sea.

This is as far as I walked, the start of the Te Waewae Lagoon. The actual mouth of the Waiau River is another 6 or 7 kilometres to the north-west.

Looking along the gravel bar that has created the Te Waewae Lagoon

Gravel cliff just at the soulth-eastern end of Te Waewae Lagoon

Detail of the gravel cliff. Very likely to one of the intermediate sources of stones for this beach and further south-east

There seemed to be more slightly larger and less rounded stones here than at Gemstone Beach, and I did not see as many colourful ones. I also found no hydrogrossular garnets although there were fossil worm cast stones.

Well-defined fossil worm casts in a large stone, left on the beach

A large dark red jasper

Probably a stone of volcanic origin

Gogeous black, grey and white stone, with fault lines, just a little too big to tumble polish

Large colourful stone

Maybe a kind of argillite

Another large stone with fossil worm casts, though they do not show the segmentation pattern as the previous one

Probably a stone of volcanic origin

I collected quite a few stones on the beach but later discarded many of them after careful re-examination. This was partly because I found much better stones later at Gemstone Beach and on the Riverton beaches. I still ended up bringing home 2.3 kilograms of stones from the beach between McCracken’s Rest and Te Waewae Lagoon.

Sorting through the stones picked up at the beach below McCracken’s Rest

A quartz, at first glance very similar to an agate

Breccia stone

Probably a quartzite stone

The fossil worm casts in this stone are raised, being made of harder material than the rest of the stone

Before returning to Riverton, I drove up to Fishing Camp Road, about two and a half kilometres north-west of McCracken’s Rest, and drove along it to the shores of the Te Waewae Lagoon. This brought me to the landward side of the lagoon, near a handful of fisher huts and a boat ramp. The stones there were dirty and slimy and uninteresting – but one could gaze across the lagoon at the gravel bar separating the lagoon from the sea and see the kind of interesting ones to be found between there and Gemstone Beach.

Just up the road from McCracken’s Rest, towards Tautapere

The beach from McCracken’s Rest to the Waiau River – Fishing Camp Road turns off the Orepuki-Tuatapere Highway (99) halfway between. Source: Google Maps.

Fishing Camp Road leads to fisher huts on the landward side of the Te Waewae Lagoon. Source: Google Maps.

Te Waewae Lagoon, fisher hut to right, boat ramp to left. Waiau River mouth out of sight in the distance

The stones on this side of the lagoon are muddy and slimy.

Looking back along the shores of the lagoon towards McCracken’s Rest (out of sight). I had earlier walked to the spot where the cliffs come down at the head of the lagoon.

Looking across the lagoon to the gravel bank, beyond which is Te Waewae Bay

Gravel bar, Te Waewae Lagoon

Some Recently Polished Stones from Riverton

001 — Just out of a 3lb borax burnishing tumble, 43 newly-polished stones from a Riverton beach.

I collected these stones at a beach I call the “Beach Past the Back Beach” at Howells Point, Riverton, in March 2019. When you reach the end of the road at Howells Point, there is a track up the sand dunes. Down the other side is this beach, which stretches for maybe 900 metres further westwards. Many of these stones were found at the far eastern corner, the area circled in this photo:

000a — Riverton’s “Back Beach” is the stretch along the last part of the road that winds along the coast. If you walk over the dunes from there, you come to the “Beach Past the Back Beach”.

These are not particularly spectacular stones, though some are really interesting. Not all have polished perfectly, some have scratches and holes in them, but I polished them because of their intriguing colours and patterns.

Photos of a selection of these newly-polished stones:

Some “Gems” from Gemstone Beach

Since I got back home from Southland at the end of March, I have been tumble-polishing stones collected from Gemstone Beach near Orepuki (see here for an account of their collection). On 28 April the polishing process finished for one 4lb barrel containing a mix of different types of interesting and colourful stones. These are the subject of this Post.

These stones are not “gems” in the sense of precious and sparkling jewel-like stones such as diamonds and sapphires. But I actually find them to be much more interesting and intriguing, many of them a mystery as to how they got the way they are. In this sense they are the true gems of Gemstone Beach.

This batch of 95 Gemstone Beach stones started their tumbling in 320 grit and then were tumbled in pre-polish and pro-polish tin oxide before a final week in a burnishing tumble with borax. This is how they ended up:

95 Gemstone Beach stones just out of the final Borax tumble

To look at some of these stones more closely…

One of them is a gorgeous quartzite (my identification of it as a quartzite is based on what I learned at Vince Burke’s Museum at Birdlings Flat):

Quartzites are not as common along the south coast as at Birdlings Flat in Canterbury, but I always find a couple of excellent ones at Riverton and Orepuki. They polish well, often have great colour, have interesting veins through them, and there’s a depth to them that makes them intriguing.

Two of the stones look like marble. Again, I can usually find one or two such stones on a collecting expedition, and I have started to keep an eye out specifically for them:

Marble-looking stone #1

Other side of marble-looking stone #2

Patterns in marble-looking stone #2

Marble is limestone subjected to heat and pressure. Veined and patterned marble is often created when a pure white original marble is cracked or shattered and the spaces between the fragments are filled with other materials. As I noted, I don’t know whether these stones are marble or not, but they certainly remind me of it. Note: April 2025 – These stones are indeed NOT marble but are iron-stained quartz. They tumble polish well and often have interesting patterns.

Over the last three years, I have found maybe half a dozen stones like this one:

I have always suspected that they are petrified wood. However, I am beginning to wonder whether they are in fact a type of jasper. I have always found them in association with dark red jasper stones, at Waikaka and on the south coast, and sometimes paler forms of jasper have been nearby. They don’t have the obvious characteristics of petrified wood, like wood grain, but it has been their light brown colour that seemed wood-like. And they have a brittleness like jasper has. Irrespective of what they are, I like them. Note: April 2025 – These stones are indeed jasper, not petrified wood.

This next stone has a very unusual set of linear features:

It could be quartz with inclusions of some other mineral. Another stone with a different kind of “inclusion” is this small green one which has specks of pink:

The next four stones have various kinds of fascinating patterns. The first stone has a breccia section at the top. The second stone is about three-quarters red with various small inclusions. The third has alternating light and dark layers plus a white halo. The fourth is actually one of my three or four favourites in this group of 95, due to its patterns of shades of grey (the last four photos below are of this stone, and they do not do it justice).

One of the most unusual stones I picked up off Gemstone Beach in March is this one:

I think these are coral fossils in this stone. I have not done much reading on fossils but I have been recently been going through a book which I found in my local library, “Rocks and Fossils” by Arthur Busbey, Robert Coenraads, David Roots and Paul Willis (2007). On page 211, the authors mention that “corals are among the most common fossils on Earth”. They refer to “rugose corals” whose characteristics could account for some features of the fossils in this stone. Rugose corals have the following structure:

Source: https://fossillady.wordpress.com/2013/01/01/favosite-corals-named-after-two-michigan-cities/rugose-coral-morphology-diagram

A fossilised coral like this would leave circular impressions of different sizes in a stone, depending how far each individual coral is eroded away from its top. Something like this would account for the patterns on the stone (above) I found on Gemstone Beach. Note: April 2025 – I suspect this stone is not fossil coral but rather an orbicular variety of some rock.

Sometimes I come across stones with pink in them. It is thought that the presence of manganese or cobalt could cause this. For example, rhodochrosite and rhodonite owe their pink colour to manganese, and cobalt ore is often pink. Here are two pink stones found on Gemstone Beach in March: Note: April 2025 – These are thulite.

Finally, five other interesting and colourful stones:

All of these stones illustrate the wide range of stones that can be found on Gemstone Beach that tumble polish well.

DEVELOPMENTS IN MY STONE COLLECTING AND POLISHING STRATEGY

I have become aware that I have changed my approach to collecting and polishing stones over the past three years. Initially, I collected any stone that looked like it might be interesting. I often thought to myself, “Let’s see what happens to this one.” I then tried to produce a completely smooth polished stone, with no flaws. I put each stone through all the stages of tumbling, starting with 100 grade silicon carbide grit. This meant that the stones lost a lot of their material by the end of the process. Along the way, I also discarded stones that had pits or cracks that were not worn away – I was looking for a perfect polish.

More recently, over the past year or so, I have come to collect much more selectively, and I polish with more leniency. When picking up stones on the beach, I now discard many of them even though I know they will polish well. They include, for instance, white quartz and red jasper, excellent examples of which I already have. I aim to collect the most unusual and interesting ones, and the ones that have very few pits or cracks, if any. I then sort through these stones when I return to my accommodation at the beach, dry them and inspect them carefully – flaws show up much more clearly when the stone is dry. I tend to discard maybe one-quarter to one-third of the ones I have collected so the ones I bring home to polish are much more likely to be interesting and to polish successfully. And I can start tumbling most of these stones with 320 grade grit, skipping two stages and saving at least two weeks of total tumbling time.

I am also less severe in judging an acceptably polished stone. I will now be happy with a stone that still has some flaws, that is not completely smooth. This means that I am able to keep a wider range of stone types. Also, the end product will be a bit bigger as it has not been worn away as much. A stone is kept based on its interesting colour or pattern rather than its smoothness.

TumbleStone Calendar 2019 – June, July, August and September

TumbleStone Calendar 2019: June

TumbleStone Calendar 2019: July

The June page for this calendar (above, left) has a photo of Kiritehere Beach, on the west coast of the North Island, which Petra and I visited in September 2018, and some of the rocks with monotis fossils that we found there. The rocks on this beach are full of these fossils. I have not tried to tumble polish them. Budleigh Salterton features on the July page (above, right). This village in Devon has a pebble beach full of red iron-stained quartzite stones from the cliffs nearby. We have visited there when we have been in Devon in the past three years. For more detail on Budleigh Salterton and its stones, see the comments on Stone #7 in the post on Twelve Stones, Part Three.

For August, one of my favourite stones appears, banded rhyolite. Stones of this type can be found on beaches along part of the south coast of the South Island, especially around Riverton and Orepuki. The beach featured on the August page is the beach past the Back Beach at Riverton (just beyond the end of the road). September’s beach is another one in the UK that Petra and I visited in 2018, Penmon Point on the Isle of Anglesey in Wales. There I found a black stone with interesting patterned fossils in it (I have not been able to identify the fossils), and a number of limestone pebbles also with fossil shells that are much fainter.

TumbleStone Calendar 2019: August

TumbleStone Calendar 2019: September

Also see TumbleStone Calendar 2019, TumbleStone Calendar 2019 – February, March, April and May and TumbleStone Calendar 2019 – October, November and December.

Twelve Stones, Part Four

Part One of this Series, about 12 stones, can be found here. The four Posts in the Series each describe the characteristics and origins of three different stones. This Post describes Stones #10 (from Riverton), #11 (from Birdlings Flat) and #12 (from Hokitika).

1-6 stones 222 7-12 stones 222

10) Stone #10 Whitish Quartzite stone with Clear Silica Inclusions, found at Riverton.

I have explained the origins and characteristics of Quartzite in relation to Stone #7 and have provided photos of Quartzites from Birdlings Flat in relation to Stone #8, both in Post Three. Quartzite stones can be found in a number of locations in the South Island. They can have a wide range of colours. One of their frequent characteristics is the clear inclusions of silica running through the stone. Quartzites polish outstandingly, due to their hardness and colours.

For the location of Riverton, see the end of the section on Stone #2 in Part One.

11) Stone #11 Banded Agate, found at Birdlings Flat.

Malcolm Luxton is a collector of Agates. He lives in Ashburton, close to New Zealand’s greatest agate-bearing locality of mid-Canterbury. In his excellent book, “Agates of New Zealand” (2015), he provides hundreds of photos of cross-sections of Agate rocks and stones he has collected from throughout New Zealand.

Cover of Malcolm Luxton (2015) “Agates of New Zealand”, Agate Orphanage Press, Ashburton.

Moss agate, Woolshed Creek, Canterbury – Page 33 of Luxton’s (2015) “Agates of New Zealand”

Banded agate, Stew Point Station, Canterbury – Page 91 of Luxton’s (2015) “Agates of New Zealand”

Banded agate, Rakaia Gorge, Canterbury – Page 230 of Luxton’s (2015) “Agates of New Zealand”

Agate in dark rhyolite, Woolshed Creek, Canterbury – Page 192 of Luxton’s (2015) “Agates of New Zealand”

Coloured Canterbury beach agate – Page 254 of Luxton’s (2015) “Agates of New Zealand”

Luxton offers this explanation for the origin of Agate:

An agate…is a stone that formed as a secondary filling in the various shaped cavities generated or forged within…lava flows. Initially these cavities were created by trapped gases or liquids or by geological processes that produced cracks or ruptures. The secondary filling permeated the lava as a silica-rich solution, and…solidified inside these cavities, thus replicating the shape of its magmatic womb. If additional materials were present, either in the silica-rich solution or growing in the cavity prior to its introduction or introduced as by-products from the surrounding lava, those minerals may have been responsible for structures, assemblages and colours (page 16).

Luxton notes that much Agate material is discharged by Canterbury rivers into the sea and some of it is cast back onto more than 100 kms of Canterbury beaches. The movement of Agates by northerly coastal currents “accounts for the notoriety of Birdlings Flat as an agate-collecting destination” (Luxton, 2015, page 250). He then refers to Vince Burke’s collection in the Birdlings Flat Gemstone and Fossil Museum.

Wall display of rocks and their origins, including Agates,Birdlings Flat Gemstone and Fossil Museum

Agate shelf display, Birdlings Flat Gemstone and Fossil Museum

Coloured Agate cross-sections, Birdlings Flat Gemstone and Fossil Museum

Large Agate rock, Birdlings Flat Gemstone and Fossil Museum

Birdlings Flat Gemstone and Fossil Museum

Upon my visits to Birdlings Flat, I have collected a few Agates, mainly quite small ones, though my wife Petra is much better at spotting them than I am! Some of the Agates I have found are just plain cloudy silica, some are banded to some extent, some have a little colouring.

Some of the Agates I have collected at Birdlings Flat

Most of the Agates collected are small, with only the occasional larger one

A “blue” Agate stone

A nice moss Agate stone from Birdlings Flat

The only Agate with bright colour that I have found

Some Agates have a whitish tinge on the outside

Banded Agate

Nice little banded Agate from Birdlings Flat

For the location of Birdlings Flat, see the end of the section on Stone #8 in Part Three.

12) Stone #12 White Quartz, found at Hokitika.

Quartz is originally a clear crystal. When subject to heat and pressure, and when water is introduced into it, it becomes white in colour, the colour of the common white “Quartz” stone found plentifully on beaches in New Zealand. However, strictly speaking, this is “Quartzite” rather than “Quartz”, though the latter term is the most common for this white stone. For that reason, and in order to distinguish it from Stones #8 and #10, I will call it “Quartz”.

Southland Museum display on Quartz. Top centre is a clear piece of Quartz

The clear Quartz in the Museum display

When walking beaches, it is the bright white Quartz stones that seem to claim the first attention of the collector’s eye. I have collected and polished many white Quartz stones from the beaches of Southland, Canterbury and the West Coast. And it is the West Coast where perhaps the best examples can be found, perhaps due to the closeness to the Quartz source, in the Southern Alps.

Dinner plate full of polished white Quartz stones

Part of a batch of Hokitika stones in the process of being polished

Held up to the light, some white Quartz stones are semi-translucent

Stone #12 was found on the beach just on the south side of the Hokitika River, less than five kms from Hokitika town. I visited there on a dull, cold, rainy day in June 2016. The beach was quite sandy but receding waves revealed drifts of stones. There were signs of coastal erosion, with flax plants being undermined and uprooted, and the end of the gravel road had been washed away.

Hokitika on the West Coast of the South Island

Signs of coastal erosion near Hokitika. June 2016.

Driftwood on the beach.

Stones being revealed by the receding waves.

Drifts of stones on the beach near Hokitika.

Mouth of the Hokitika River, looking north at Hokitika on the far shore.

A large white Quartz stone found at Rapahoe Beach, just north of Greymouth, on the same trip. I had to leave this there as I was travelling home by plane and taking other stones with me.

What Hokitika is most famous for – its greenstone carvers.

Location of Hokitika (click on image below, left), and location of beach near Hokitika where Stone #12 was collected (below, right) (source: Google Maps):

This brings us to the end of this series of Posts.

Twelve Stones, Part Three

This Post describes Stones #7 to #9 of the 12 (Part One dealt with Stones #1 to #3 and Part Two with Stones #4 to #6):

7-12 stones 222

7) Stone #7 Red-Stained Quartzite, found near Budleigh Salterton, England.

On two recent visits to England, in 2017 and 2018, I went to a coastal village of about 5,000 inhabitants called Budleigh Salterton in Devon, not far from Exeter. The Doomsday Book records that salt panning was of great importance here, and it may have been as far back as Roman or even Iron Age times, hence the origin of the name “Salterton”. The beach, three miles (nearly five kilometres) in length, has a distinctive mix of stones, many deriving from the Budleigh Salterton Pebble Bed which outcrops in the cliffs there.

Looking south-west towards the village of Budleigh Salterton

Looking north-east along the beach at Budleigh Salterton

Budleigh Salterton beach huts

Beach huts and red cliffs at Budleigh Salterton

The layers of pebbles found in the cliffs at Budleigh Salterton originated over 400 million years ago when sandstones formed under desert conditions in the place in France we now call Brittany. During the Triassic period, these rocks were eroded and transported by vast rivers across what later became the English Channel to form the Budleigh Salterton Pebble Beds. The pebbles then fell onto the beach and have been transported eastwards by the sea, well beyond Budleigh Salterton.

The East Devon and Dorset Coast, known as the Jurassic Coast, has become a World Heritage Site due to the geology of the area. In 95 miles (150 kms), 185 million years of earth history are on display in the coastal cliffs. Older Triassic rocks (between 251 million and 200 million years old) give way to Jurassic rocks (200 million to 145 million years old) and younger Cretaceous rocks (145 million to 66 million years old).

Part of an Information Panel in Budleigh Salterton

Information Panel section on the pebble beds

Red pebble cliffs

Dry stones on Budleigh Salterton beach

When the stones are wet, their colours become apparent. In the middle is a stone with dark red speckles, just like Stone #7

Stone #7 is at bottom centre, just off the beach

The British geologist Ian West has a very interesting web page on Budleigh Salterton and the surrounding area (even though it can be difficult to read in places). There he states that the dominant pebbles in the Budleigh Salterton Pebble Bed are quartzite (if you go to this link, click again on it to enlarge the writing at the bottom of the photo). Quartzite is a very hard durable metamorphic rock composed almost entirely of quartz. It forms when a quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism. These conditions recrystallize the sand grains and the silica cement that binds them together. The result is a network of interlocking quartz grains of incredible strength. Impurities in Quartzite can cause it to be yellow, orange, brown, green, or blue. The presence of iron results in pink, purple or (as with Stone #7) red.

Location of Budleigh Salterton in south-west England (source: Google Maps):

Budleigh Salterton location

8) Stone #8 White-Grey Quartzite.

I collected this stone at Birdlings Flat, Canterbury, a beach that I try to visit at least a couple of times each year because of the Quartzites, Jaspers and Agates (see Stone #11) to be found there. Birdlings Flat is the beach immediately south of Banks Peninsula, part of Kaitorete Spit, an enormous barrier gravel bank which lies between Lake Ellesmere and the sea. Stones washed down from the Alps by Canterbury rivers are swept northwards along the coast and deposited here.

Where Birdlings Flat beach meets with Banks Peninsula

Looking southwest along Birdlings Flat beach

The waves are powerful and unpredictable, especially at the Banks Peninsula end

The Birdlings Flat beach is part of the 25 km long Kaitorete Spit which consists entirely of stones

Looking for stones about eight kilometres along Kaitorete Spit

As mentioned in relation to Stone #7, Quartzite is a very hard durable metamorphic rock composed almost entirely of quartz. Many different coloured Quartzite stones can be found at Birdlings Flat, as attested by the collection to be found at the Birdlings Flat Gemstone and Fossil Museum, which I have visited a number of times.

Part of the Quartzite Collection at the Birdlings Flat Gemstone and Fossil Museum. At top right is a large white-grey stone, with a fault through it. Photos with permission

Some white-grey Quartzite stones similar to Stone #8 can be seen on the lower row

Another part of the Quartzite Collection at the Birdlings Flat Gemstone and Fossil Museum

More Quartzite stones, Birdlings Flat Gemstone and Fossil Museum

Stones in Birdlings Flat Gemstone and Fossil Museum, including Quartzites

Stone #8 is one of a number of white-grey Quartzites I have found at Birdlings Flat. The large bands of white and grey provide a striking contrast, making this stone particularly attractive.

Stones collected at Birdlings Flat, including some white-grey Quartzites

Two Quartzites from Birdlings Flat

Two good-sized tumble-polished Quartzites from Birdlings Flat

A darker white-grey Quartzite

Quartzite with more white than grey

A white-grey Quartzite found and polished by Petra

There are a number of previous TumbleStone Posts about Birdlings Flat. These include: Birdlings Flat Gemstone and Fossil Museum; Seven Types of Stones Collected at Birdlings Flat; Selection of Online Sources about Birdlings Flat; and Information on Birdlings Flat from Books.

Location of Birdlings Flat (source: Google Maps):

Birdlings Flat location

9) Stone #9 Unknown stone, found at Riverton.

I picked up this interesting little stone at Henderson Bay, Riverton Rocks. I initially wondered if it might be a piece of petrified wood, given its pattern and texture. Some agatised wood (see photos below) that I have seen has some similar patterns. Petrified wood forms when wood is buried by sediment and protected from decay by oxygen and organisms. Groundwater rich in dissolved solids flows through the sediment, replacing the original wood material with, for example, silica, calcite, or pyrite. Agatised wood has been petrified by agate, a form of chalcedony or microcrystalline quartz. However, the agatised wood I have seen tends to be quite glassy when polished, something absent from this stone.

Agatised wood specimens at the Birdlings Flat Gemstone and Fossil Museum

Agatised wood, from a YouTube clip https://www.youtube.com/watch?v=Rxc1j6QNPvI

In March 2017 I bought a piece of petrified wood from Hettie’s Rock and Crystal Shop in Akaroa on Banks Peninsula.

Petrified wood bought at Hettie’s, Akaroa, March 2017

Hettie’s Rock and Crystal Shop, Akaroa

Another side of the same piece of petrified wood

I have also seen petrified wood at Curio Bay, about 130 kms east of Riverton.

Curio Bay, Catlins Coast, Southland

Information Plaque about Curio Bay

The steps down to Curio Bay

Looking down on Curio Bay from the top of the steps

Trunk of a petrified tree at Curio bay

Petrified wood in the rocks at Curio Bay

Petrified wood in the rocks at Curio Bay. My foot for scale

Petrified wood in the rocks at Curio Bay

On the basis of what I have seen, I am unsure whether Stone #9 is agatised wood or some other form of petrified wood, or something entirely different. I have more unidentified stones than identified ones in my collection. [Later I came to the view that this stone is much more likely to be a mix of quartz and a black mineral, not any type of petrified wood.]

I found Stone #9 at Henderson Bay, the last of the Bays that make up Riverton Rocks, the part of Riverton that is made up of holiday homes. My grandparents owned a “crib” (“bach” to North Islanders of New Zealand) at Henderson Bay, up on the hill overlooking the sea, and when I was a boy my family spent two weeks every summer on holiday there.

Henderson Bay, Riverton Rocks, September 2018 – the road continues on to the Back Beach

Henderson Bay, February 2016

Henderson Bay, looking back at the steps

Argillite rocks at Henderson Bay, stones above the high tide mark on the beach

Drift of stones, Henderson Bay

Henderson Bay stones

Location of Henderson Bay and Back Beach, Riverton (source: Google Maps):

location Riverton bays333

Part Four is the final Post in this series, dealing with Stones #10, #11 and #12.

Twelve Stones, Part One

I recently presented 12 of my polished stones to a work colleague in appreciation.

zDSC01427

I took on the job of Assistant to an AB Technician in the dairy industry for three weeks, at the height of the insemination season. This was a completely new experience for me and I enjoyed it a lot, largely due to the support and patience of Tony, the Technician I was assisting. I chose to give him 12 stones because I spent a lot of time preparing the 12 inseminator implements that he regularly used. Below is a photo of inseminators being loaded (a straw of bull semen is loaded into the end of each inseminator, its plug snipped off with scissors, then a plastic sheath is threaded over it to hold in place).

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These are the twelve stones:

1-6 stones 222 7-12 stones 222

I am taking the opportunity in this Series of Posts to dig into aspects of different types of stones and the places from which they were collected. This first Post will describe Stones #1 to #3, with the rest dealt with in Parts Two, Three and Four.

1) Stone #1 Dark Red Jasper

I found a rock in April 2016 near Waikaka in Eastern Southland. It was embedded in a farm track on “The Mains”, the farm on which I grew up. Red Jasper is a rock to be found as part of the Waikaka quartz gravels which contain the gold mined in the area between the 1880s and 1920s. There is an old gold dredging pond about 100 metres north of the track. I extracted this large Jasper rock and took it home with me to Karapiro on the plane. Later I broke off a few pieces to tumble-polish. Due to the brittle nature of the rock, I did not completely smooth the stones out before their final polish. (Click on the photos below to view them and see their accompanying captions.)

The farm track on “The Mains” – when it reaches the trees, it crosses the Waikaka River on a bridge. I found the Jasper rock about 30 metres from this point.

I broke a number of pieces from the Jasper rock in order to tumble polish them. I used a geological hammer (and I wore gloves and safety glasses while doing this), The pieces were sharp and a bit brittle.

The Jasper rock embedded in the track.

After being dug out and washed.

Not a good colour as this was taken inside, with poor lighting, but this gives an idea of the size of the Jasper rock.

A number of the Jasper fragments before being tumble polished.

After tumble polishing for about five weeks. Stone #1 is in the top row, far left.

Location of Waikaka at the bottom of the South Island (source: Google Maps):

Waikaka location

More on Jasper as a gemstone.

Other TumbleStone posts referring to “The Mains”: Maps as a Resource: New Zealand’s “Maps Past”, Part One; Maps as a Resource: New Zealand’s “Maps Past”, Part Two; Jasper Stones and Petrified Wood, Shepherd’s Creek, Waikaka; and “You run from the river, when it long ran over you…”

2) Stone #2 Granite or Basalt Porphyry, containing Feldspar Crystals

I found this stone at Riverton, Southland, in September 2018. This type of stone, while not numerous on Riverton beaches, is not rare and one is likely to be found within 10 to 15 minutes of searching. Its most obvious and interesting characteristic is the lighter-coloured feldspar crystals packed into the darker surrounding matrix. This indicates the stone is of volcanic origin, and is probably either basalt or granite. “Porphyry” is a term for the texture of an igneous rock consisting of large-grained crystals, such as feldspar or quartz, lodged in a fine-grained silicate rich matrix. These crystals formed within magma underground – this material then reached the earth’s surface and cooled relatively quickly, “trapping” the crystals in fine-grained rock. The crystals can vary in size, as shown in these three photos from the Riverton Museum “Te Hikoi” (these photos were taken with the permission of Museum staff in December 2016):

Riverton Museum (photo taken with permission)

Riverton Museum (Photo taken with permission)

The Riverton Museum often has a display of local stones types, usually set up for children. The well-known geologist Hamish Campbell has been instrumental in putting this together. In 2017, the Museum developed a five-page pamphlet as a guide for children and families to collect nine different types of stones from the area between Riverton and Orepuki.

Riverton Museum Guide Children’s Rockhound Club Adventure p.4

Riverton Museum Guide Children’s Rockhound Club Adventure p.2

Rock Type 1 “Volcanic Rocks” would include Granite and Basalt Porphyry. Reference.com provides the following explanation of the differences between Granite and Basalt: Granite is an intrusive rock that is formed when magma cools inside the crust. This slow cooling gives time for crystals to grow, making it more coarsely grained than an extrusive rock, formed at the earth’s surface. Basalt, on the other hand, is an extrusive rock, with a smooth texture from rapid cooling, usually by water. Granite has a high silicon content; it is mostly made out of quartz, mica and feldspar. Basalt contains more calcium oxide, manganese oxide and iron compounds than granite. I am unable to exactly identify Stone #2. Granite stones can easily be found in the Riverton area (see Stone #4) but Stone #2 seems to be less coarsely grained than Granite.

Riverton’s Back Beach, on Foveaux Strait‎, a source of many interesting stones for polishing

Further to the west than the previous photo. The Back Beach lies just beyond the far-off point.

Riverton has many small beaches full of stones

Location of Riverton (source: Google Maps):

Riverton location

3) Stone #3 Unknown Variegated Green Stone

This stone, found at Riverton in February 2018, is darker green towards the thicker end, with the other end being a lighter green. When slowly rotated in the light, small specks of mica can be seen at the darker end. It is probably a metamorphic stone having been subject to significant heat and pressure at some stage. So I looked up metamorphic rocks in “A Photographic Guide to Rocks & Minerals in New Zealand” (2011), written by Nick Mortimer, Hamish Campbell and Margaret Low. Page 20 (see below) has a basic classification which led me to consider whether at least part of this stone could be “Hornfels”. Page 119 (also see below) mentions that Hornfels contains “small micas” and that some occur in the Bluff area, which is along the coast not far south-south-east of Riverton.

“Photographic Guide to Rocks & Minerals of NZ” page 20

“Photographic Guide to Rocks & Minerals of NZ” page 119

Many of the stones on Riverton beaches are thought to have come down the Waiau River whose mouth is about 50 kms westwards.

Stones are often unique in their shape, size and patterns, and contain a range of elements, layers, intrusions and so on, which make them very difficult to identify with certainty. Chemical analysis is often needed. Stone #3 remains unidentified.

Stones #4 to #6 are described in Part Two.

The Seven Stages in Tumble Polishing Stones: The Stones After Pre-Polish and Their “Inspection” and “Sorting” Before Stage Six

(This is the ninth Post in this series – the first Post can be found here.)

Two of the stones have suffered some minor damage in the Pre-Polish tumble, probably from bumping into other stones. This is despite careful handling and the use of plastic beads to cushion collisions in the barrel. This will happen from time to time due to the brittle make-up of some stones. Stone 6 has lost a small chip in its side:

001b - Copy

Stone 12 has has lost three or four small pieces, the largest being apparent in the photo below:

002

In such cases, stones are usually returned to Stage Two or Three, to be re-tumbled with 220 or 320 grit to get rid of the pits. However, I will put them through to the Pro-Polish stage for the sake of this series of Posts.

The following are the stones as they looked upon initial collection from the beach at Riverton and then upon completion of Stage Five, Pre-Polish. In both instances, the stones are dry. Note that the same sized segment of graph paper is provided in these comparative photos. At present the stones look a lot smoother and shinier, and are slightly smaller. Their colours are clearer and brighter, with any patterns being more obvious.

Stones 1 to 5:

1st 5 dry a 1st 5 after pre p

Stones 6 to 10:

2nd 5 dry b 2nd 5 after pre p

The grain of Stone 6 is now apparent. The wearing away of Stone 7 has led to the band across its lower part to be uncovered more and thus made to appear larger.

Stones 11 to 15:

3rd 5 dry b 3rd 5 after pre p