The 2025 TumbleStone Calendar features beach stones collected during my two South Island fossicking trips in 2024. Details about the stones shown in the months of January and February are here, in March and April here, and in May and June here. The month of July is another featuring Southland’s Gemstone Beach and August takes us back to Slope Point, first encountered in the month of February. The stones in this Calendar were photographed in the rough while wet, held in my hand in bright sunlight. Often the photos reveal more about the stones than I noticed when I picked them up on the beach. Also note that my identifications of stone types are generally tentative, and should not be relied on unless previous Blog Posts provide good evidence for them.
JULY – GEMSTONE BEACH
Gemstone Beach is right next to the small town of Orepuki in Southland. It lies 70 kilometres west of Invercargill. The mountains of eastern Fiordland can be seen from it, further to the west. The beach is on the Te Waewae Bay coast, on the shores of Foveaux Strait. The name “Gemstone Beach” most narrowly applies to a 200 to 250 metre stretch of beach in front of the carpark opposite the “Gemstone Beach” road sign. However, I use the name to refer also to the beach that runs from the carpark to the mouth of the Waimeamea River, 1.5 kilometres to the northwest. July’s beach photo, featuring a female Paradise Shelduck, was taken not far from the Waimeamea River on 8 September during a three hour morning fossick that was my first on Gemstone Beach for that trip.
July’s Stone 1 is a trace fossil in argillite. One large trace fossil dominates the stone,seen from the side, but there are a number of other light-coloured “patches” in the stone, some of which are likely to be trace fossils viewed from the top (or bottom).
The local name for this trace fossil is a fossil “worm cast” as it looks a bit like what a worm throws up on the surface after soil has passed through the worm’s body. The trace fossil is what has been left behind after a small animal has burrowed through the sediment at the bottom of the sea or has crawled along its surface about 250 to 300 million years ago. It’s probably not been left by an earthworm. The shape of the trace, a line of chevrons, is known as “protovirgularia”. More information can be found in “I” is for “Ichnogenus Protovirgularia” and “The Tattooed Rock, The Trace Fossils… Revisiting Gemstone Beach’s Trace Fossil Stones”. Argillite is a hardened mudstone, one of New Zealand’s basement rocks. See University of Auckland Geology for a brief account.
I found July’s Stone 2 on Gemstone Beach on 18 February. I have labelled it a quartzite but it might be a brecciated quartz.
What I find most appealing in stones like this is the delicate tracery of mineral colour through it.
I have suggested that Stone 3 could be “Rondonite” but that should read “Rhodonite”, my only spelling mistake in the early versions of this Calendar. Rhodonite is a pink manganese silicate mineral usually associated with black manganese oxides which may occur as dendrites, fracture-fillings, or matrix within the stone (Geology.com). Jocelyn Thornton has three examples of rhodonite on page 26 of “Gemstones”. It is the pink and black combination that we use to identify rhodonite and to distinguish it from the likes of thulite, another pink stone with manganese in it (see Stone 4 in the month of April).
Te Ara The Encyclopedia of New Zealand notes: “Rhodonite, a pink manganese mineral, is found as lens-shaped deposits within schist in Otago and the Southern Alps. It usually occurs naturally with a rind of black, oxidised material called pyrolusite.” The best quality and most well-known rhodonite in New Zealand is found near Akatore Creek, south of Dunedin. The Otago Rock and Mineral Club for many years held the mining rights to this source. I have found a couple of stones on Gemstone Beach that could be rhodonite, July’s Stone 3 being the best specimen, found on 20 March this year. I note that Marion Troon’s tray of stone specimens from Gemstone Beach includes a rhodonite – see about halfway though the September 2024 “The Guardian” article on Gemstone Beach.
Stone 4 is a small poppy jasper (orbicular jasper), with some hematite in it, that I found on 22 September.
Compared to January’s Stone 3, another poppy jasper, the “poppies” or orbs are quite small, consisting of tiny dots. According to Wikipedia, “In highly silicified rhyolite or tuff, quartz and feldspar crystallize in radial aggregates of needle-like crystals which provide the basis or seed for the orbicular structure seen in this kind of jasper.” For more information, see “O is for Opaque Orepuki Orbicular Jasper”. A great diversity of orbicular jasper exists across the globe, as can be seen in Mindat (the images below come from there).
The poppy jaspers I have found on Gemstone Beach, and that other people find elsewhere in New Zealand, compare well within such diversity.
AUGUST – SLOPE POINT
We visited Slope Point in this Calendar in February. In her booklet, “Gemstones” (1985), Jocelyn Thornton has an entry on Slope Point. She writes on page 35: “On the Southland coast between Waipapa Point and Haldane there are a few pebbly beaches visited by fishermen and rock collectors. The cliffs contain layers of conglomerates with pebbles which weather out and collect on the beaches. At first glance they appear dark, but closer examination reveals a multitude of subtle colours and patterns.” The photo of the beach below was taken during a visit there on 13 March, which is when this month’s Stone 1 was collected. The beach photo was taken from under the high cliffs to the north of the bay, looking across the pebble beach and solitary holiday home to the walking track that slopes down to it.
Stone 1 is an example of a type of stone that I have found a few times here. On the early print runs of the Calendar, I labelled it as “Rhyolite”. However, further thought led me to change that to “Volcanic Breccia”, though most if not all the fragments in it are likely to be rhyolite.
I initially thought that the small white bits in the stone were likely to be crystals, hence the thought that it was rhyolite. However, there are clearly larger bits (clasts) in the stone as well, some consisting of alternating dark and light bands. These will have come from different pieces of stones. As I noted in relation to February’s Stone 2, also a Slope Point breccia, a “breccia” consists of small pieces of rock (clasts) that have not been rounded much by transport by, for example, water before being cemented together. This contrasts with a “conglomerate” which consists of more rounded clasts indicating some erosion before being cemented together and thus having travelled further before formation. The clasts may be the result of faulting (even small movements in rocks caused by tectonic pressure) or a landslide or a volcanic eruption. Of course, some stones can consist of both sharp-edged and rounded clasts, confusing the categorisation!
20 November: I have just finished tumble-polishing Stone 1. The result is very good, with a significant clarification of the stone’s features and a permanent shine.
There are large conglomerate boulders on this beach, some of which can be seen in the foreground of the beach photo. These have rolled down the cliffs from the conglomerate strata that jut out here. Stone 2 is a very small conglomerate stone, found on 20 September – and there are photos in the 20 September Post showing the cliffs.
This is a “polymictic” conglomerate, where the clasts have come from more than one rock type – see Alex Strekeisen‘s description of types of conglomerate. In Stone 2, some of the tiniest clasts seem to be angular, not rounded like the larger ones. As far as tumble polishing goes, it may turn out that the clasts are not well enough cemented to survive much tumbling, and tiny fragments may fall out of it.
August’s Stone 3 was my most intriguing find of 2024. I could tell when I found it on 20 September that it has interesting little bits inside it but it wasn’t until I photographed it that a wondrous sight presented itself.
It looks like the stone consists of hundreds of tiny fossils. The mushroom-like shapes in it (long stalks with a funnel at one end) especially caught my eye (see photos below). There appear to be bits and pieces of different white fossils there. I have not seen anything like this before.
I posted photos of Stone 3 on the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”, asking if anyone could identify it. Most people did not know what it might be, though one person made a very helpful suggestion: “It looks to be agatized forest floor – composed of leaf litter, stems, and small bits of wood.” That seemed reasonable to me. Jocelyn Thornton, on page 24 of “Gemstones” , presents a stone that she describes as follows: “Peat or other plant debris may be preserved in silica. The example is a chunk of stems, roots, leaves, and so on from Southland, where this material is called ‘forest floor’.” I think I have seen such stones before, but they have not had the strange features within them as are present in Stone 3. Nevertheless, in the early printing runs of the Calendar, I labelled Stone 3 as “Agatised Forest Floor?”
Then, while writing up the description of May’s Stone 3 from Ward Beach, I followed up a suggestion by “Kween Bee”, in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”, that the tiny elongated features in it could be fossil sponge spicules. What are spicules? The 2017 NIWA publication “Splendid Sponges” states on page 4: “Most sponges produce a skeleton of fibre made from a special collagen called spongin, which may or may not contain sand grains or spicules. Spicules are siliceous elements made by the sponge that come in an amazing array of forms and are usually used to identify the species. Some sponges have only spicules and no spongin.” As Wikipedia puts it: “Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge’s skeleton and thus it provides structural support and potentially defense against predators.” There are three major groups of sponges, some of which have calcium carbonate spicules (Class Calcarea, currently rare in New Zealand), and some of which have silica spicules (Class Hexactinellida and Class Demospongiae). I decided that I couldn’t really find any spicules similar to what is in May’s Stone 3 from Ward Beach, but then I wondered whether they might be relevant to August’s Stone 3 from Slope Point. I found a couple of diagrams of spicules in Wikipedia which didn’t show anything similar to Stone 3’s long-stalked mushroom-like shapes (see first two diagrams below). Then I came across an academic article in “Aquatic Biology” in 2020 in which a diagram of spicules showed something similar to Stone 3’s feature (see third diagram below).
The spicule labelled B in this third diagram comes from a sponge called “Acalle recurvata” which is to be found in the Amazon River in Brazil. That’s a long way from Slope Point and it’s not a fossil. But I find it suggestive that maybe Stone 3 contains fossil spicules of some kind. So in the later versions of the Calendar, Stone 3 is labelled “Fossil sponge spicules?” and some more research sometime is in order.
Stone 4 is a kind of spherulitic (or orbicular) rhyolite that I found on this Slope Point beach on 2 March.
These are not common and are keenly sought after. I have found maybe three or four. The circular features in the stone are cross-sections through small spheres with a radiating structure that have formed in the rhyolite. In “Some More Recently Polished Stones from Slope Point and Kakanui”, the second stone is a slightly different spherulitic rhyolite in which radiating arms are apparent. These are sometimes called chysanthemum rhyolites because of their flower-like appearance. Alex Strekeisen writes: “Spherulites are typically two-mineral aggregates (mainly quartz and feldspar), formed by initial spherulitic growth of one mineral and later crystallization of a second mineral from the liquid or glass between the fibres.” Gordon Sherwood (New Zealand Rocks Downunder) has a YouTube video of hunting for spherulitic rhyolite near Waihi. From 9 minutes 9 seconds in the video, he shows some good specimens in the rough.
The months of September (Gemstone Beach) and October (Ward Beach) in the 2025 TumbleStone Calendar are featured here. This 2025 Calendar is for sale, with postage within New Zealand included, for $30 non-rural, $35 rural. Contact me at john.tumblestone@gmail.com.
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