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 2227-12 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.

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.

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.

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”.

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. 

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.  

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. 

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). 

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.

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.

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.

There are a number of previous TumbleStone Posts about Birdlings Flat. These include: Birdlings Flat Gemstone and Fossil MuseumSeven Types of Stones Collected at Birdlings FlatSelection 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 wonder 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. 

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

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

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.

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. 

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 Two

I recently presented 12 tumble-polished stones to Tony in appreciation for his support when I undertook work with him in the dairy industry. Part One of this series of Posts described Stones #1 to #3. This Post deals with the characteristics and origins of Stones #4, #5 and #6, and later Posts will examine Stones #7 to #12. 

These are the 12 stones:

1-6 stones 2227-12 stones 222

4) Stone #4 Pink Granite

This stone was found at Riverton in February 2018. Granite is a coarse-grained igneous rock, made up of quartz, feldspar and mica. Its grains are large enough to be seen by the unaided eye. Pink Granite is given its colour by a larger proportion of potassium feldspar. Granite is found exposed at various places on Stewart Island and the western part of the South Island. It can be seen along the coast between Colac Bay and Orepuki in Southland, and stones of Pink Granite can often be found on southern beaches. Click on the photos below to see their captions.

5) Stone #5 Finely Banded Rhyolite(?)

This stone was found at Orepuki, Southland, on Gemstone Beach in April 2016. I have collected a number of such stones before on southern beaches and find them fascinating as they conjure up the image of cosmic gas trails in faraway starfields. A fellow stone collector once told me they were Rhyolite but it is difficult to confirm this from the various online and published sources on Rhyolite, maybe due to the distinctive pattern on these particular stones.

Rhyolite is very similar in chemical composition to Granite. But while Granite has crystals that are generally easy to see, in Rhyolite the crystals are often too small to see. This is due to the more rapid cooling of the Rhyolite lava at the earth’s surface compared to granite’s slower cooling magma within the earth.

Examples of other Rhyolite stones found at Orepuki or Riverton:

Orepuki is a small village on a sparsely settled part of the southern coast. Gold was mined in the district during the last two decades of the nineteenth century, and in the 1880s the southernmost Chinese settlement in the world was to be found there. The beach just to the north of Orepuki is called Gemstone Beach – it is said that semi-precious gems such as garnet, sapphire, jasper, quartz and nephrite can be found on the beach, along with hydrogrossular stones and argillite stones containing fossil worm casts. (The garnets, sapphires and nephrite are extremely rare.) Gold can also be found in patches of black sand at low tide.

Location of Orepuki (source: Google Maps):

Orepuki location

6) Stone #6 Marble(?)

I found this stone also on a Riverton beach. I don’t know what type of stone this is, but it reminds me of Marble. Marble is formed out of limestone which is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite and in its pure form is white. However, it can also contain other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite, which provide colouring. Veined and patterned Marble, as this stone could be, is often created when a pure white original Marble is cracked or shattered and the spaces between the fragments are filled with other materials.

Stones #7 to #9 are described in Part Three.

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).

aaa

These are the twelve stones:

1-6 stones 2227-12 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.)

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 OneMaps as a Resource: New Zealand’s “Maps Past”, Part TwoJasper 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):

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.

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.

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. 

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.