March 2020 – TumbleStone

Stay-at-Home Day Six, Tuesday 31 March 2020: Stone Six

Stone One.

Stone Two.

Stone Three.

Stone Four.

Stone Five.

Above are the first five stones already presented in this Series. The first Post is Stay-at-Home Day One.

The stone in the photo below can be found at Gemstone Beach and Riverton. It is not plentiful and sometimes I might find only one or two during a week of beach visits although at other times I can collect six or seven good specimens.

Stone Six: Ignimbrite stone from south coast of South Island.

Stone Six: Polished banded ignimbrite stone.

I came across a small number of these stones during my early stone collecting trips about three years ago and they caught my attention because of the banding and colour. They conjure up for me images of deep space and galaxies and stars, and the rings of planet Saturn.

Carina Nebula, 8,500 light years from Earth. Source: https://en.wikipedia.org/wiki/Nebula

Artist’s concept of the Milky Way Galaxy. Source: http://spaceref.com/astronomy/the-milky-way-ransacks-nearby-dwarf-galaxies-of-their-gas.html

Saturn’s rings. Source: https://www.everythingselectric.com/saturn-rings/

Blast wave from a supernova. Source: https://en.wikipedia.org/wiki/File:SNR_0509.jpg

Although the stone is hard and glassy in character, it can be hard to find specimens that are not cracked or chipped, and sometimes there are tiny gaseous pockets that prevent a smooth surface. I have been fortunate to find some that have polished well.

It is the presence of tiny gaseous pockets (see close-up photos below) that gives a big clue that this is a volcanic rock. It is probably ignimbrite. Ignimbrite originates as the deposit of a pyroclastic flow, which is a hot suspension of particles and gases flowing explosively and rapidly from a volcano. New Zealand geologist Patrick Marshall originally came up with the term in the 1930s meaning “rain of fiery rock dust” (from the Latin “igni” for fire and “imbri” for rain). Some ignimbrite can be very loosely deposited, with lots of pumice in it. If it is buried at depth, it becomes compacted, gases and liquids are squeezed out, and it becomes solid and fine grained and glassy.

Stone Six.

Stone Six detail.

Initially I identified these stones as rhyolite, another volcanic rock, based on information provided by a couple of fellow rockhounds. When I emailed photos of the stones to a New Zealand geologist, he generously provided me with a detailed analysis of them, suggesting they were a type of ignimbrite. He wrote: “My guess is that they were eroded from areas of the latest Jurassic/earliest Cretaceous Loch Burn Formation of eastern Fiordland” and came down the Waiau River to the coast.

I have found some large specimens of this stone, especially at Gemstone Beach. The one below left I left on the beach but I managed to get the one on the right home though it is too big to be tumble-polished:

Found on Gemstone Beach, 10 May 2019 – left on beach.

Brought home, early 2019.

I have already polished a number of ignimbrites and there are more awaiting polishing in the future (see photos below). The Whanganui Rock and Mineral Club was scheduled to have a Show next month and I had planned to develop a display of Gemstone Beach stones with ignimbrites being one of three featured groups. However, as you can guess, the Show has been cancelled – we are all still going to be staying-at-home on 18 April.

Ignimbrite stones awaiting polishing.

Polished ignimbrite stones.

Polished ignimbrite intended as part of a rock club display.

The next Post in this Series is Stay-at-Home Day Seven.

Stay-at-Home Day Five, Monday 30 March 2020: Stone Five

Stone One

Stone Two.

Stone Three.

Stone Four.

Above are the first four stones already presented in this Series. The first Post is Stay-at-Home Day One.

Stone Five, from Gemstone Beach, appears at first sight to be a white stone with streaks of brown throughout:

Stone Five: Polished stone from Gemstone Beach.

Stone Five: Other side of polished stone from Gemstone Beach.

However, closer inspection reveals that this is composed, like Stone One, of many small fragments held together in a fine-grained matrix:

Close-up of Stone Five from Gemstone Beach.

Stone Five.

The fragments look like quartz. This stone would be a “breccia” if the fragments were angular but would be a “conglomerate” if they were rounded due to abrasion from the actions of water or wind. Breccias form close to the site where the constituent fragments have been created, while conglomerates form further away, after the fragments have travelled somewhat and had their angular edges rounded off. It seems to me that most of the fragments are angular with only the occasional one showing some abrasion. This leads me to identify it as an example of a breccia.

The next Post in this Series is Stay-at-Home Day Six.

Stay-at-Home Day Four, Sunday 29 March 2020: Stone Four

The first Post in this Series is Stay-at-Home Day One.

Stone Four is a very unusual stone. I have not come across one like it before and it is a bit of a mystery to me.

002 — Stone Four: Polished stone from Gemstone Beach.

003 — Stone Four: Other side of polished stone from Gemstone Beach.

I found Stone Four on Gemstone Beach on 9 June 2019 – I know it was 9 June because I took a photo of it on that day and the date is listed in the photo’s properties. Its appearance then was quite different:

001 — Stone Four on the day it was found on Gemstone Beach

The stone as found on the beach had a rough surface with light brown patches across it. But under those patches seemed to be a slab of a glass-like texture. In some ways, the smoothness of that underlying texture reminded me a bit of the hydrogrossular garnets which can be found on Gemstone Beach but hydrogrossulars don’t have this kind of surface. In other ways, the stone’s underlying glassiness reminded me of the kind of agate stones found on Birdlings Flat in Canterbury but agate is extremely rare on the south coast and Stone Four feels heavier than agate.

When held up to the light, it is apparent that there is a large black inclusion filling the right-hand side of the stone. In fact, this inclusion comes to the surface at the top of the stone, one-third of the way along from the right in the following photo (it can also be seen as a black patch in the second photo at the top of this Post):

006 — Stone Four held up to the light, revealing an inclusion in the right-hand side.

My guess at this stage is that the glassy part of Stone Four consists of a kind of cryptocrystalline quartz, maybe chalcedony. Quartz is the second most abundant mineral in Earth’s continental crust, behind feldspar. It is composed of silicon and oxygen atoms. Cryptocrystalline quartz is made up of crystals too small to be seen, and it often looks like porcelain. Chalcedony is the general name given to this kind of quartz and with the additions of colours and patterns it makes up a wide variety of rocks, including carnelian, agate and jasper. The lack of any pattern and the overall colour of the glassy part of Stone Four leads me to suggest that chalcedony could be the appropriate identification for it. [May 2022: I now believe this is likely to be hydrogrossular garnet, after comments from members of the Facebook Group, New Zealand Lapidary, Rocks, Minerals, Fossils.]

The next Post in this Series is Stay-at-Home Day Five.

Stay-at-Home Day Three, Saturday 28 March 2020: Stone Three

The first Post in this Series is Stay-at-Home Day One.

Stone Three was found on Gemstone Beach in 2019. I don’t know what kind of stone it is apart from probably being a type of metamorphic stone, that is, it has undergone transformation by heat and pressure. There are a wide range of types of green stones to be found on beaches in New Zealand, many of them quite spectacular in pattern and complexity.

001 — Stone Three: Polished stone from Gemstone Beach.

002 — Stone Three: Other side of polished stone from Gemstone Beach.

Close-ups of the stone reveal the complexity of its colours and patterns:

Stone Three detail.

Stone Three.

Next Post in this Series is Stay-at-Home Day Four.

Stay-at-Home Day Two, Friday 27 March 2020: Stone Two

Stone Two was found on the Back Beach at Riverton last year, at the end of the road, on the shores of Foveaux Strait, with Rakiura/Stewart Island in the background. I think it is a quartzite stone, with lace-like tendrils spread throughout. As I have stated in a previous Post, 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.

A quartzite starts off as a quartz-rich sandstone, a sedimentary rock that is grainy and feels like sandpaper. When the sandstone is exposed to high temperatures and pressures, the hard glassy metamorphic rock of quartzite is formed. Quartzite’s wide variety of colours are a result of minor amounts of impurities being incorporated with the quartz during the process of metamorphism. It is often the cloudy or lace-like trails of impurities that provide the most fascinating aspect of a quartzite stone.

Stone Two: Polished quartzite stone from the Back Beach, Riverton

Stone Two: Other side of polished quartzite stone from the Back Beach, Riverton

Stone Two detail.

Stone Two.

Stone Two detail.

Stone Two.

Stone Two detail.

Stone Two.

Next Post in this Series is Stay-at-Home Day Three.

Stay-at-Home Day One, Thursday 26 March 2020: Stone One

Today New Zealand entered Covid-19 Alert Level Four, the highest Alert Level for dealing with the Covid-19 pandemic. This means that everyone is to stay at home, apart from those in essential services, initially for a period of four weeks. People are allowed to travel to supermarkets and pharmacies to obtain the necessities of life but are otherwise to remain at home, interacting only with those in the same household. The aim of this unprecedented measure is to break the chain of virus transmission within the community and avoid a catastrophic overwhelming of the health system. As at Wednesday 25 March, New Zealand officially had 205 confirmed cases of people with Covid-19, only six of them requiring hospitalisation but none in Intensive Care. Now is a good time to take extreme measures to minimise the impact of future cases.

To mark the passing days of Alert Level Four, I will choose a polished stone each day. Fortunately I am also able to continue to tumble stones throughout this time of restriction – I have some good stores of stones yet to be tumbled. Each day, I will post photos of a recently polished stone.

Stone One, collected on Gemstone Beach in 2019, is a breccia with small mainly white and light grey fragments in a grey matrix. A breccia is a sedimentary stone made up of randomly oriented angular fragments cemented together in a fine-grained matrix, the fragments not previously eroded by water or wind. Like this stone, it can be similar to a piece of concrete.

002 — Stone One: Polished breccia stone from Gemstone Beach

001 — Stone One: Other side of polished breccia stone from Gemstone Beach

Stone One.

Stone One detail

Stone One

Stone One detail

Next Post in this Series is Stay-at-Home Day Two.

Birdlings Flat Stone Collection Trip, February 2020

I spent five days based in Christchurch in late February, which enabled me to visit Birdlings Flat beach four times. The sea was very calm over this time and the weather posed no difficulties. At times, I was the only one on the beach or there were only two or three others there, walking a dog or fishing or also looking for interesting stones.

Clouds over Banks Peninsula, viewed from Birdlings Flat.

Birdlings Flat on a calm day.

Birdlings Flat, 24 February 2020.

A dog walker in the faraway. Birdlings Flat, 21 February 2020.

Three seagulls, Birdlings Flat, 21 February 2020.

Driftwood construction, Birdlings Flat, February 2020.

Looking through a driftwood construction, Birdlings Flat, February 2020.

On one of the days, during a weekend, a group of photographers were at the Banks Peninsula end of the beach, making the most of the cliffs, waves and sky. I was accompanied that day by Diane, my niece and host. As we neared the photographers, we came across an unusual creature on the stones – it looked kind of like a sea caterpillar, but I have not been able to identify it.

Unidentified beach creature.

Photographers, Birdlings Flat.

Unidentified beach creature.

I collected just over 9 kilograms of stones to take home to polish. I found a few beach agates, one of which had some very nice bands. I also saw some of my favourite yellow quartzite stones, including a couple of larger ones that had to be left on the beach.

An excellent banded agate found on Birdlings Flat, 24 February 2020.

The banded agate in amongst the stones on Birdlings Flat.

A different agate collected on Birdlings Flat, February 2020.

Large yellow quartzite stone, Birdlings Flat, February 2020.

In between visits to Birdlings Flat, Diane and I drove an hour inland and took a jet boat trip up the gorge of the Waimakariri River. We were fortunate enough to be the only two customers for Alpine Jet Thrills for that trip and had two guides to show us the landscape features. The geology of the gorge was interesting, though the rocks swept down the river to the sea end up north of Banks Peninsula and would not make their way to Birdlings Flat.

Diane (right) and our two guides/drivers from Alpine Jet Thrills, Waimakariri River.

On the Waimakariri River.

Geological formations of the Waimakariri River gorge.