A Blog About Stone Gathering, Tumbling and Polishing, and Rocks and Landscapes, from New Zealand – With Musical Interludes (john.tumblestone@gmail.com)
I am a member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In 2021, the Administrators invited members to submit photos each week of a stone, mineral or fossil in their collection that started with a letter of the alphabet. I posted my entries on TumbleStone Blog – see here for the Index to them. Recently, some members of the Group thought it would be a good idea to do it again. Here is my first entry (slightly revised and extended) for the 2025 Series. (In 2021, I posted “A is for Amygdaloidal”.)
“A is for Agatised Fossil Bone”
I found this stone on a beach near Kakanui, North Otago, in September 2024. I had not seen anything like it before, nor since. It has been suggested that it is agatised fossil bone. You can view similar material if you google “dinosaur gembone”. The process that produces it is called “permineralisation”, where the empty spaces within a buried organism, spaces filled with liquid or gas during life, become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process is well described in Geologyin.com (though sometimes the ads can be annoying on this website). I thought about whether to tumble polish the stone for a while and eventually decided to do so.
After tumble polishing.
After tumble polishing.
After tumble polishing.
I tried some gentle tumble polishing: a couple of short tumbles in 400 grit, then a 600 grit stage followed by a polish powder tumble. I did not have spectacular success (see photos above) – it looks like the material varies significantly in hardness, parts of it resisting taking a shine.
NOTE: A couple of members of the group, from the local area, mentioned they had similar though non-agatised stones, identified as fossil whale bone.
On Thursday, two days ago, I had my final Gemstone Beach fossick for this trip. Friday was spent packing up and driving to North Otago, a trip of more than four hours. I arrived in Kakanui, a few kilometres south of Oamaru, just before dark, staying at a holiday home booked through AirBnB. This morning, I spent two hours at Seadown Beach, a couple of kilometres north of Kakanui village. I had called in at this beach on 16 February, on my way south.
It was a very foggy morning though the sun eventually came out. Previously, a lot of sand had been covering most of the stones here. There’s still a lot of sand but the sea has removed a depth of about two metres of it across the south end of the beach, leaving a small cliff stretching for about 50 metres there. The removal of this sand has exposed a 100 metres stretch of stones close to the waves.
Seadown Beach this morning, as viewed from the roadside parking area.
The 2 metre high cliff in the sand at the south end of the beach.
Lots of stones at the south end of the beach, at low tide, by the rocks.
Seadown Beach stones, Kakanui.
The type of stone I keep the sharpest eye out for on this beach is one I first discovered in mid-2022 (see here, last part of Post). It is a dark brown colour, similar to the limonite prase found along the North Otago coast. Jocelyn Thornton describes limonite prase as an “ochre-yellow jasper” (page 34 in “Gemstones”). Two examples of limonite prase:
Limonite prase, found at Kakanui’s Seadown Beach in June 2022.
Limonite prase, found at Kakanui’s Seadown Beach in March 2024.
In June 2022, I had found a stone at Seadown Beach that reminded me of limonite prase, was a dark brown colour but did not have the typical black patches. Looking at it more closely, I noticed some very faint tiny markings in the stone. Taking a photo of it later, I zoomed in on the markings and discovered a wonderland of tiny figures:
Found in June 2022.
I posted these photos on the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. John Taylor, an Englishman who lives in Scotland but often visits New Zealand and is a member of the Group, commented that he had found a very similar rock in Europe, a “Danish Fossil Okkergelber Hornstein”. He pointed out that this rock is chalcedony and contains many tiny bryozoan fossils. When John visited me in 2023, he gave me a slice of a rock he had found at Waabs in northern Germany, just south of the Danish border, on the shores of the Baltic Sea:
Slice given to me by John Taylor.
The Waabs area on the Baltic Sea coast, Germany. Source: google Maps.
I took photos of John’s slice, wet and in bright sunlight – the tiny bryozoan fossils can be seen in the close-ups:
Side A of slice.
Side A, close-up 1.
Side A, close-up 2.
Side B of slice.
Side B, close-up 1.
Side B, close-up 2.
I have since found more of these stones on Seadown Beach. Three from visits in 2023:
Found in January 2023.
Found in May 2023.
Also found in May 2023.
The following is what I have found from some initial research into bryozoa (the term “bryozoans” is also used). In her book “The Field Guide to New Zealand Geology” (2003), Jocelyn Thornton describes bryozoans as one-celled animals (zooids) which live in colonies (page 52). Most marine bryozoans have a partially calcified, hard, body wall, which is subject to the process of fossilisation. There are many species and many different types, including branching twig-like forms, lacy forms and fan-like forms. A 2022 news article notes they are often known as “moss animals”, because the bushy-looking live ones look like moss, the literal meaning of bryozoa from the Greek, bryon meaning moss, zoön meaning animal. They are also known as “sea mats” or “lace coral”, depending on the variety and the form they take. The excellent NIWA identification guide, “Bountiful Bryozoans”, available free online, refers to individual zooids as ranging in size from 0.3 to 1.5 millimetres long, only just able to be seen with the naked eye “if you look very carefully”. New Zealander Dennis Gordon is a world authority on bryozoa and co-author of a field identification guide. In a 2003 “New Zealand Geographic” article entitled “Living Lace”, he notes that 960 of the 1500 living species of bryozoa occur in the New Zealand region, the highest regional “bryodiversity” in the world.
Worldwide, there are about 15,000 named fossil species. According to Wikipedia, mineralized skeletons of bryozoans first appear in rocks from the Early Ordovician period(470 to 485 million years ago). The website “Fossil Finds in New Zealand” includes images showing the different forms that fossil bryozoa may take (see first photo below).Gordon reports an abundance of bryozoan fossils in New Zealand limestone: “It has been estimated that Te Kuiti limestones, including those in the Waitomo area, are 50 to 80 per cent bryozoan remains, and Oamaru stone 70 per cent.” The second photo below is of a sample of Oamaru limestone. Ernst Haeckel’s drawing of bryozoa skeletons, published in 1904, are dramatic renderings of their tiny appearance (see third photo below). He also produced a drawing of live bryozoa (Plate #23 in his “Art Forms in Nature”, published 1899 – fourth photo below).
Haeckel (16 February 1834 – 9 August 1919) was an influential German zoologist, naturalist, and artist, who discovered, described and named thousands of new animal species (he is also controversial due to some of his political and racial views). Haeckel’s published artwork includes over 100 detailed, multi-colour illustrations of animals.Modern scanning electron pictographs of bryozoa fossils provide a similar close-up view of their tiny skeletons.
I have yet to look more carefully at the tiny figures in the Seadown Beach stones I have found, to see how they match up with different varieties of bryozoa. And there may be other types of tiny fossils in them. I have also yet to confirm whether the stones are chalcedony or not.
To return to my visit to Seadown Beach today. I found two stones that clearly are the same as my previous finds and very similar to John Tayor’s slice:
Two other finds are less clear. They are similar in colour and both have tiny forms in them, but the forms may not be bryozoa:
The tiny forms in this stone may be bryozoa??
Not sure that this stone contains bryozoa.
Six other finds from this morning of a roughly similar colour but they are either limonite prase or yellow-brown stones of different compositions but no bryozoa:
Limonitte prase.
Limonite prase, with an interesting central patch mosaic.
This stone also has a kinf of mosaic patch to the left.
A kind of breccia?
The vein may be chalcedony?
I picked this up, thinking it might be a quartzite.
Among the other interesting stones I found today was a piece of seam agate.
Held up to the sky.
With a torch behind it.
Other side of seam agate.
In March 2021, I found a piece of seam agate a few kilometres south near Moeraki Village – see the second half of the Day 20 entry in this Post.
Other finds this morning included a number of quartzites, reasonably common on this beach:
Side A of light-coloured quartzite with intriguing clouds..
Side B of same stone.
Light-coloured quartzite with tiny clear quartz inclusions.
Quartzite with a range of hues.
A draping quartzite??
Quartzite with an intense brown mineral.
Veined quartzite.
A differently-veined quartzite.
My final five finds today follow. The last two are too soft and crumbly to tumble-polish but are specimens of Kakanui Mineral Breccia. As one source notes: “The Kakanui Mineral Breccia is a mineral-rich fragmental rock which formed from the contact of an intruding magma with water about 31.6 million years ago (Early Oligocene). The resulting violent eruption totally disrupted the magma components into angular fragments.” There is international geological interest in the breccia for a variety of reasons. More information on Kakanui Mineral Breccia can be found in this Post, about halfway through, in the section on Kakanui Beach.
Interesting mix of breccia and crystals. Side A of small stone.
Side B of stone.
Fossilised sea floor stone.
Another fossilised sea floor stone.
Kakanui mineral breccia with maybe some white chalcedony in it.
Anther Kakanui mineral breccia stone.
The next Part of this Series reports on my last fossick at Kakanui for this trip. The Series Index is here.
From a tumble-polishing point of view, today’s stone has to be treated a little differently. It needs only a 400 grit tumble to reach its most shiny state. Taking it further does not improve it, I have discovered. Such a stone comes from the sediments and shells and tiny animals that fall to and accumulate at the bottom of the ancient ocean, close packed and petrified.
There is an amazing diversity of little things in this type of stone, and many times, when examining close-up photos, I can spot something I didn’t see before:
Other side of Stone #13.
The stone was found on a beach just north of Kakanui on 24 June 2020 and came out of the 400 grit tumbler in November 2021. The stone as found on the beach (see below) clearly showed what it consisted of, though some fossilised sea floor stones have a more weathered surface that masks the detail. The 400 grit tumble removed some of the surface, changing to some degree what can be seen:
Stone #13 when originally found on the beach.
Other side.
About five years ago, while in England, we bought a set of fossil stone table coasters. They came from the Lyme Regis Fossil Shop in Devon, England. We also have a candle holder and egg-shaped stone from the same material.
Fossil stone egg, coaster set and candle holder.
Fossil stone coaster set.
Fossil stone coaster bought overseas.
Fossil stone coaster bought overseas.
I have lost the details of their origin but a couple of websites suggest it is “the foothills of the Himalayan Mountains”. This website refers to it as “Himalayan marble”. I never thought I would find stones to rival this in New Zealand!
For more examples of these Kakanui stones on TumbleStone Blog, see here.
Stone of the Day #14 is here. The Index to the January 2022 Stone of the Day Series is here.
Here are four Kakanui stones that have been tumbled only in 400 grit. I have discovered that tumbling them further does not improve them. As it is, they come out of the 400 tumble smooth and waxy-feeling, and showing clearly the often-tiny fossils within them. I have previously referred to them as fossilised sea floor – they are a kind of limestone.
The four fossil sea floor stones from Kakanui after 400 tumble.
The beach just north of Kakanui village where I found these stones.
Another view of the Kakanui beach.
Stone 1.
Stone 2.
Stone 3.
Stone 4.
When found on the beach, these stones are often unremarkable, being whitish, sometimes showing some darker grains within them, sometimes hinting at the small fossils in them, often feeling grainy and powdery. Some of them have holes in them, some of them have no fossils. But many of them turn out to be very intriguing. About 900 metres down the beach from where I park my car (at the end of Seadown Road), there are usually some rocks poking through the sand that are made of the same stuff, though I have not seen the tiny fossils in them that these stones contain.
Fossil sea floor rocks 900 metres down the beach.
Examples of “rough” fossil sea floor stones from Kakanui, as found.
I tumbled a batch of these stones in 400 grit in a 3lb barrel recently. I did not open the barrel until after 11 days of tumbling. Inside was a thick fine-grained slurry and I worried initially that the stones might have been worn down too small. It turned out they were still of a good size – I have usually tumbled them only for about 7 days, and in future will restrict them to 6 to 7 days in the barrel. These four stones were among the most interesting in that batch – the tumbling revealed much more than was able to been seen in the “rough” stones. Here are the first two stones:
Stone 1.
Stone 1 close-up.
Stone 1, Side B.
Stone 1, Side B, close-up.
Stone 2.
Stone 2 close-up.
Stone 2 close-up.
Stone 2, Side B.
Stone 2, Side B, close-up.
Stone 2, Side B, close-up.
Here are Stones 3 and 4 – Stone 4 is less grainy and maybe more dense than the other three:
Oliver Simpson found this stone on a Slope Point beach at the bottom of the South Island. When he first showed it to me, I could see nothing in it. He then pointed to some very faint markings and mentioned “coral”. I don’t know whether the stone warmed in my hand and as a result showed the outlines of the coral septa (radiating plates as seen from above) more clearly, but I could then start to make out the faint shapes distinctive to coral.
Oliver’s fossil coral, before tumbling.
Oliver’s fossil coral, before tumbling.
Oliver’s fossil coral, before tumbling.
A previous Post on fossil coral can be found here.
Oliver’s stone is a rare and enigmatic stone, rare because it is fossil coral, enigmatic because it does not seem to show itself easily in person. Oliver and I discussed whether it would be worth tumbling the stone, to bring out the coral shapes more clearly. I was reluctant to do anything to spoil such a rare specimen, but in the end I agreed to give it a go. I tumbled it in a very fine 600 grit for 9 days to see if any improvement occurred.
After 600 grit tumble.
After 600 grit tumble.
Removal of a very thin surface layer brought out the coral shapes in a brighter white. However, I was not keen to try more as I did not think further “improvement” or clarity could be achieved. So the stone then went into a pre-polish then a pro-polish tumble:
The end result is still enigmatic, and lacks some of the subtle beauty of the original stone. While the shapes of the coral can be seen more clearly, they appear harsh and not always cleanly defined.
Recently the Facebook Group I belong to, “New Zealand Lapidary, Rocks, Minerals, Fossils”, invited its members to submit photos each week of a stone, mineral or fossil in their collection that starts with a letter of the alphabet. My Posts for the first two letters can be found here. The following are my next two Posts in the series.
“C” is for “(Fossil) Coral” – I found this fossil coral stone on Gemstone Beach (Orepuki, Southland) on 10 June 2020. My sister was showing me a stone she had found when I looked down and saw this stone with faint unusual markings. Not only the top of the corals can be seen but also the view from the side. I have not seen anything like it before on Gemstone Beach, nor since.
Top of the polished coral fossil stone.
Side view of polished coral fossil stone.
Top view of fossil coral stone when found on Gemstone Beach.
Side view of fossil coral stone when found on Gemstone Beach.
Of the fossil corals, there appear to be two types (families) which this stone could belong to – rugose and scleractinian. They both have “septa”, the radiating segments seen from the top. But the symmetry in rugose is bilateral, meaning that two identical halves can be created, while scleractinians have radial symmetry (reflecting two different types of growth processes). See Palaeo Post. The well-known Petoskey stone, the official stone of the US State of Michigan, is a rugose coral – the photo of a polished Petoskey stone below is from http://www.geologyin.com/…/what-is-petoskey-stone-and… I have also posted below two diagrams from Wikipedia, showing the rugose (“tetracorallia”) and scleractinian (“hexacoralla”) corals from a 1904 book.
Entry for a rugose coral on page 29 of “A Photographic Guide to Fossils of NZ”
To my eyes, my stone looks closer to the rugose family but I am far from certain, given the great diversity within each family type. The book “A Photographic Guide to Fossils of NZ” has an entry (p.29) for a rugose coral (see above) (one of a total of six entries for coral fossils in the book). The stone I found on Gemstone Beach has tumble-polished well, with just one small shallow hole to interrupt its smoothness.
I failed to anticipate how much attention this Facebook Post would receive from the members of the “New Zealand Lapidary, Rocks, Minerals, Fossils” Group. The week after the Post, it was given 53 “Wow”, 78 “Love” and 176 “Like” along with more than 50 Comments about its beauty and interest. The Administrators of the Group decided to make it the Group banner for a while.
I was not happy with the photos I had used for the Facebook Post, the close-ups in particular looked too yellowish. So I took some more photos and tried to produce better close-ups. I wrote: It is a difficult stone to photograph as it is curved, and focus plus lighting are difficult (I use my camera’s automatic settings). I have tried again but don’t know if these are much of an improvement. Creating the close-ups seems to accentuate the yellow tinge, and I have tried to adjust for this, without much success. However, interesting aspects of the coral structure can still be seen.
A few days later, I was searching for information on the geology of Slope Point, to provide some context for the rhyolite stones I found there. In one book I consulted, “The Natural History of Southern New Zealand”, I came across a reference to fossil coral at Te Waewae Bay. Gemstone Beach is located towards the eastern end of Te Waewae Bay. The following is stated on pages 62 and 63 of the book, in Ewan Fordyce’s outstanding chapter on “Fossils and the History of Life”: “Te Waewae Bay rocks yield the only specimens of a bizarre pseudo-colonial form of sceleractinian coral apparently related to Flabellum… [It sits in the] siltstone of the Te Waewae Formation; probably upper Kapitean, latest Miocene; western Te Waewae Bay.”
Te Waewae Bay coral fossil, related to Flabellum. Page 63, “The Natural History of Southern NZ”
Note: “The Natural History of Southern New Zealand” is edited by John Darby, R. Ewan Fordyce, Alan Mark, Keith Probert and Colin Townsend, published in 2003 by University of Otago Press. Currently available from University of Otago Press for $80.
Coral of the Flabellum family are fan-shaped, usually solitary, and seem quite different from the fossil coral I found – see NIWA Critter of the Week., Mindat Fig. 4 (esp. A & B) and Mindat Fig 5. I have not yet come across any other reference to fossil coral found along the south coast.
To return to the alphabetical series of Posts in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” – I posted the following on 25 May 2021:
“D” is for “Dendrites” – I found this stone in March 2021, on a beach not far from my motel on the northern side of Kaikoura town, while I was looking for stones with trace fossils (zoophycos). The stone is probably a limestone(?) or mudstone(?), too soft for tumble polishing. The word “dendrite” is from the Greek word for “tree”, referring to its branching. In geology, dendrites are thin, branching crystals, often of some variety of manganese oxide (or iron), that grow over a surface in a rock or mineral. They are found in cracks or along bedding planes.
Unpolished Kaikoura stone with dendrites.
The beach at north Kaikoura where the stone was found. Kaikoura town centre is at middle right of photo.
