Month: April 2026

A Review of “AlexStrekeisen” Website

This Review originally appeared in the March 2026 edition of “StoneSpeak”, the Newsletter of the Southland Geological and Lapidary Club. Below is a slightly expanded version. An Index to this Series of Reviews is here. The Southland Club’s website is here and its Facebook Page is here.

When I’m doing some research on a rock or a mineral, I usually look at the relevant entry in the “AlexStrekeisen” website. Some of the information there is quite detailed and often accompanied by good photos of rocks.

Cover of “StoneSpeak” March 2026.
Page 7 of “StoneSpeak” March 2026.
Home page of http://www.alexstrekeisen.it/english/index.php
Alessandro Da Mommio. Source: http://www.alexstrekeisen.it/english/index.php

There are also microphotos of thin rock sections at the end of each entry – photos of what you’d see looking through a microscope. These are intriguing and can be quite spectacular:

Calcite and zeolite (grey) filling an amygdale. Field of view = 7mm. http://www.alexstrekeisen.it/english/vulc/amygdales.php
Fragments of various nature in a breccia, field of view = 7mm. http://www.alexstrekeisen.it/english/sedi/breccia.php
Olivine crystals in a dunite, field of view = 7mm. http://www.alexstrekeisen.it/english/pluto/dunite.php
Allanite crystals, biotite and microcline feldspar in a granite, field of view = 7mm. http://www.alexstrekeisen.it/english/pluto/allanite.php
Globigerinids Foraminifera in a Mudstone, field of view = 2mm. http://www.alexstrekeisen.it/english/sedi/mudstone.php
Hornblende, plagioclase and quartz in a type of gabbro, field of view = 7mm. http://www.alexstrekeisen.it/english/pluto/quartzgabbro.php

Alessandro Da Mommio is the Italian geologist behind this website (photo at top of Post, far right). He notes on the home page that his passion for optical petrography inspired him to create an online vehicle for “the recognition and identification of mineral phases in thin section”. The website is available in both Italian and English. If you find yourself on the Italian-language version of the site, click on the Union Jack flag just under the header to be taken to the English-language version.

The website is named after Albert Strekeisen (1901-1998), a pioneering Swiss petrologist who has been very influential in rock classification systems. The QAPF (Quartz – Alkali feldspar – Plagioclase – Feldspathoids) diagram for the classification of igneous rocks is known as the “Streckeisen diagram” in his honour (see image of diagram below, far left). Da Mommio is a geology lecturer at the University of Milan and a specialist in microphotography. Last year, with Victoria Pease he published the book “Atlas of Minerals and Igneous and Metamorphic Rocks in Thin-Section”.

“Streckeisen diagram”, the QAPF diagram for classification of igneous rocks. https://commons.wikimedia.org/wiki/File:Streckeisen_Plutonic_Rocks.svg
Rhodonite page. http://www.alexstrekeisen.it/english/meta/rhodonite.php
Rhodonite, Franklin Mine, New Jersey. http://www.alexstrekeisen.it/english/meta/rhodonite.php
Granular rhodonite crystals, field of view = 7mm. http://www.alexstrekeisen.it/english/meta/rhodonite.php

Da Mommio describes his website as follows: It is “designed for students of geology and petrography and geology enthusiasts… Each page consists of two parts, descriptive (an exhaustive explanation of the mineral phase) and visual (high-resolution photographs).” New pages on rocks and minerals are being added regularly – one of the most recent is Rhodonite which has especially colourful thin-section microphotos (see image above, far right).

The website has five main sections – on the four main types of rocks (plutonic, volcanic, metamorphic and sedimentary) and on the Italian magmatic provinces (areas of volcanic rocks). The rock sections have long lists of contents running down the left margins, arranged under a small number of headings. For example, for Metamorphic Rocks, the main headings are Metamorphic minerals, Texture and microstructure, and Metamorphic rocks. For Sedimentary Rocks, the main headings are Sedimentary rocks [main types], Turbidites, Fossils, Sedimentary minerals, Texture, and Rocks. For each entry under those headings, there is usually a description of the mineral, rock or item, often with technical detail.

Page on Metamorphic Rocks. http://www.alexstrekeisen.it/english/meta/index.php
Page on Sedimentary Rocks. http://www.alexstrekeisen.it/english/sedi/index.php
Page on Plutonic Rocks. http://www.alexstrekeisen.it/english/pluto/index.php

However, the entries vary in length and usefulness, some being quite detailed but others being quite short. A few of the entries rely heavily on Mindat.org, but most go beyond that. Unfortunately, the list of references for minerals and rocks seem to be mainly textbooks, not online sources that you can follow up. For that, you then need to go to such websites as Geology.com and Wikipedia.

The next Review in this Series is of three New Zealand YouTube rockhounds. The Series Index is here. 

March 2026 Stone Collecting Trip – Part 17, Low Tide at Ward Beach, Thursday 2 April

I have a booking on a Cook Strait ferry for tomorrow but the ship has been out of action for a couple of weeks. It was supposed to have started sailing yesterday but cancelled again. Today it looks like it is due to start up again and, if so, I should be lucky tomorrow. I had managed to book an alternative with another ferry company for Sunday and the Flaxbourne Motel owners at Ward, Ronnie and Ian, have very kindly made allowances for me to stay until then if need be. So today’s trip to Ward Beach may be my last fossick for this trip, or it may not.

I arrived at Ward Beach at 12.30 pm at low tide. It was sunny, 18 degrees, with only a light breeze. More than a dozen camper vans and cars were there. However, during the nearly two hours I was there, no one else was on the beach in front of the carpark or to the south, just over the Flaxbourne River, where I walked.

Looking south from in front of the Ward Beach carpark.
Looking notth from in front of the Ward Beach carpark.
At low tides, these rocks to the south are exposed.
The Flaxbourne River flows as a shallow stream here (top right). The mouth is a great fossicking area.
Looking back towards the carpark and camping area.

It was a typical Ward Beach fossick though I was surprised not to spot more trace fossil limestone pebbles. This Series of Posts features the kind of stones I have found most often on this beach. Here are 15 of my finds from this afternoon.

This first one is my favourite – a small stone but with really interesting features and colours:

Samll but busy colourful find.
Other side of stone.

Occasional jaspers can be found on Ward Beach. Many are plain and uninteresting to me, but now and again a fascinating one appears:

This jasper has a bright white vein which may be chalcedony (agate):

Jasper with white vein.

I keep a look out for limestone stones with white veins, and found these three:

This one has dendrites in it (see the second half of this Post for other Ward Beach examples):

Limestone with dendrites.
Other side of stone.

I also look for brecciated limetone (complex veins) and sometimes these might have some flint in them (the clearer brown patches in this case):

Brecciated limestone? Plus some flint?
Other side of stone.

More examples of this can be seen in this Post.

Some limestone along this coast can be yellow and/or blue. This one appears to be brecciated, has a thin black vein and an intriguing white feature:

A dark red stone that I initially thought could be a jasper but I think it is probably some kind of igneous stone – the tiny crystals and patches of mineral in it are interesting:

Igneous stone?
Other side of stone.

Another igneous stone, a type quite common on New Zealand beaches, probably a porphyritic basalt with plagioclase feldspar phenocrysts – the different crystal shapes always fascinate me:

This stone is predominantly light in colour (probably quartz) with black crystals:

Finally, three specimens of trace fossils in limestone:

That night, I met up with Katinka, Guy and their daughter, all keen rockhounds and occasional readers of this blog. They were staying two doors along from me at the motel. On my way to catch the ferry the next day, I stopped off briefly at Ward Beach again and was soon joined by them for about 20 minutes. The tide was not good for finding wet stones however and little eventuated from that fossick. I made it safely home to Whanganui that night.

The Index to this Series can be found here.

Three 2026 Calendars For Sale, Reduced Price

Details about the 2026 TumbleStone Calendar can be found here. I sold the last one in stock a couple of weeks ago, even though it is now well into 2026.

Calendar Cover, 2026.
July.
August.
September.

I have ordered three more copies in case I get another late order or three. The photos themselves can be cut out and put on the wall, if you are so inclined. April’s stones are poppy jaspers, with May’s stones being Kakanui quartzites and April feature black and white igneous stones.

April’s poppy jaspers.
May’s Kakanui quartzites.
The black and white igneous stones of June.

The price these days is 20% off the original price, with postage within New Zealand included, coming now to $28 non-rural, $32 rural. Email john.tumblestone@gmail.com.

March 2026 Stone Collecting Trip – Part 16, An Introduction to St Andrew’s Beach, South Canterbury, Tuesday 31 March

A member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” mentioned to me three weeks ago that the beach near Pareora in South Canterbury was worth a look. Pareora is about seven kilometres south of Timaru South, a beach I often visit. I had a look at the area on Google Maps and decided to drop in on a beach just south of Pareora, a couple of kilometres north of the small town of St Andrew’s. I turned off the main highway onto Galletly’s Road – it’s a very short drive down that road to a freedom camping area right next to the beach. When I arrived at 11.15 am, there were a small number of buses and campervans making use of it, some of them sporting Gypsy Fair signs. A sign warns that an old landfill used to be located at the back of part of the beach. And there are some railway iron posts sticking out of the stones in front of the beach entrance.

Parked at the entrance to St Andrew’s Beach.
Sign at St Andrew’s Beach.

I spent an hour on St Andrew’s Beach. The wind was cool, the temperature about 12 degrees, and it was mainly cloudy. There are a lot of grey stones on the beach. Initially I didn’t spot much else as I walked south along the waves’ edge. I turned back north and shortly thereafter found a small piece of petrified wood. I slowed down and began to look more carefully, soon spotting a few scattered stones worth examination. I ended up spending an hour doing this. As expected, there are a lot of similarities to the stones at Timaru South – some very interesting quartzites and jaspers with the odd unusual stone.

Looking south on St Andrew’s Beach.
Looking north on St Andrew’s Beach.
St Andrew’s Beach stones.
St Andrew’s Beach stones.

Nine of my finds are presented below. The first is the piece of petrified wood I picked up:

Petrified wood from St Andrew’s Beach.
Other side of petrified wood.

This next one is my favourite find, due to its colour and texture – I think it is most likely a quartzite:

Quartzite?
Other side of possible quartzite.

I picked up this small dark stone because I thought I could see spots in it. The close-up photos suggest the “spots” could be spherulites, and maybe the stone is rhyolite – but this is speculation based on my experience with Slope Point spherulitic rhyolite. The photos are not clear enough to confirm my thoughts:

Spherulites?

This small stone has a dark brown mineral through part of it and some brecciation in another area:

Interesting brecciation.
Other side of stone.

Maybe a quartzite, also with some mineral patches and brecciation:

“Top” view.
“bottom” view.
“Side” view.

Some fine mineral lines in this find which look like epidote green:

Fine traces of epidote in this stone?
Other side of stone.

A smaller stone with a little more of the mineral through it:

I think this one could be jasper but I am not sure:

Jasper?
Other side of stone.

Finally, a dark green quartzite of a type that I have found before along the southeastern coast of the South Island:

Quartzite.

Part 17 reports on finds from Ward Beach, the day before I arrived back home in Whanganui. An Index to this Series is here.

March 2026 Stone Collecting Trip – Part 15, Finding Patches of Stones on Seadown Beach, Kakanui, Monday 30 March

I arrived at  Seadown Beach at 3pm, just after high tide. Compared to yesterday, there were fewer stones at the south end of the beach. I decided to walk north to try to find some stone patches and eventually did so, just as I was about to turn around. It was maybe 600 metres from the roadside carpark that four or five large areas of stones appeared. These provided some interesting finds.

Looking south along Seadown Beach.
Looking north along Seadown Beach.
About 500 metres north along the beach, I spotted some stones ahead.
The patches of stones.
The stones were being wet by the waves, assisting fossicking.

It was 19 degrees, partly cloudy, with a light breeze so I didn’t need my hat and scarf. I left the beach two hours after arriving.

I picked up a number of brown stones, looking for those which contain tiny bryozoan fossils. This one turns out to be the most intriguing, looking as if it does have some tiny fossils of some kind:

One side of stone with tiny fossils.
Other side of stone.

This one also seems to have some tiny fossils in it:

Another interesting brown stone.

A small highly brecciated brown stone:

Brecciated brown stone.

A fourth stone I collected because it is brown – it has quite a different texture:

Another interesting brown stone.
Other side of stone.

Four interesting quartzites from this afternoon, all of different character:

Small colourful quartzite,
Yellow hues in quartzite.
Quartzite #3.
Different kind of quartzite, with green vein.

A brecciated white quartz:

Brecciated quartz.

Finally, I think this stone is an agate (chalcedony) but it’s not like any other agates I have found on this beach. It is opaque and contains inclusions that look like the yellow/orange iron oxide that colours many of the quartzites:

One side of agate(?), wet.
The same side dry.
Other side, wet.
Other side, dry.
Opaque when held up to light.

In Part 16, I continue my way north, stopping off at a beach near St Andrew’s in South Canterbury. An Index to this Series is here.

March 2026 Stone Collecting Trip – Part 14, Starting the Trip Back Home, A Fossick on Seadown Beach, Kakanui, Sunday 29 March

I reached Seadown Beach, just north of Kakanui, at 2pm this afternoon after a long drive from the south coast of Southland. It was raining during the first hour or two of the drive but scattered showers then dominated. At Seadown Beach, the tide was high but there were a lot of stones high up the beach, wet from the waves or from recent rain. This was especially the case among the rocks at the south end of the beach, where the stones have been piled higher than I have seen before.

Seadown Beach, Kakanui.
Looking north along Seadown Beach.
There were good pickings at the south end of the beach.

The sea must have recently been turning the stones over and churning them up because I had a very productive hour-long fossick. I found an amazing range of quartzites but a number of other very colourful and patterned stones as well, plus a piece of petrified wood.

This is the petrified wood, a relatively rare find for me on Seadown Beach:

Petrified wood.
Other side of petrified wood.

There are links to other specimens of petrified wood I have found on this beach here.

I think this next stone is what is known locally as limonite prase:

One side of this stone looks like plain quartz but the other side is far from plain:

The plain side.
The more interesting side.

A colourful orange stone with different patterns on different sides:

Another orange find, this one with some brecciation (tiny fragmentation):

These are seven of the quartzites I collected, quite a diverse lot:

I kept my eye open for the brown stones that sometimes contain tiny bryozoan fossils – this is the best candidate but I can’t clearly see such fossils in it:

And finally, an interesting mix of colours:

I return to Seadown Beach tomorrow. An Index to this Series is here.