TumbleStone Calendar 2025: May and June

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 and for the months of March and April are here. The month of May is the first of two months featuring Ward Beach in Marlborough (the other month is October). June focuses on Kakanui’s Seadown Beach (which we first met back in March). Note that the stones in the Calendar were photographed in the rough while wet, held in my hand in bright sunlight. The photos often reveal more about the stones than I noticed on the beach. My identifications are often missing or tentative, and should not be relied on unless previous Blog Posts provide good evidence for them.

MAY – WARD BEACH

May’s photo of Ward Beach was taken at low tide, looking northwest towards the carpark. When the tide comes in, the rocks in the foreground become completely covered. I took the photo when visiting the beach on 2 October, on my way home at the end of my second fossicking trip for the year. I found Stones 3 and 4 below on that day. Ward Beach is about six kilometres off the main road east 0f the small village of Ward in Marlborough, 82 kilometres north of Kaikoura and 45 kilometres south of Blenheim. I had two fossicks that day and the photo was taken at 10 in the morning when the tide was low.

The first Ward Beach stone for May is what I think could be a veined limestone pebble with some brecciation. Often the veins are bright white, as in this one, found on my way south early in September.

I wonder if the brown bits in the stone might be flint. A wide range of types and colours of limestone can be found on Ward Beach, many of them veined and/or brecciated, some with flint inclusions. See, for example, the Series “Selection of Tumbled Ward Beach Stones”, where Part 2 features “Limestone Pebbles” and Part 3 “Stones With Veins”. In Thornton’s “Gemstones”, page 31 “Beach pebbles – East Coast from Hawke Bay to North Canterbury”, there are specimens of such limestone.

May’s Stone 2 is most likely a jasper, found on 25 March, the final fossick of my first trip. This year, I have started to pay more attention to the red jaspers on Ward Beach.

The printing of the Calendar makes this stone look red when, as you can probably see here, it actually has an orange tinge (at least, it does to my eye). When I noticed this, I double-checked what photo I had used, and confirmed that it had turned out differently. This is the only case where it appears the printing process has significantly changed the colour of a stone in the 2025 Calendar. I have not struck this before.

Stone 3 is my most exciting find on Ward Beach so far, though I am struggling to identify it. It may not be a good candidate for tumble polishing but it has fascinating detail in it. I found it on 2 October, the last fossick of the year. What caught my eye were what I thought were the elongated crystals within it.

“Acicular” crystals are thin and needle-like, either tapering to a point or having a blunt termination. A “prismatic” crystal habit is pencil-like, with elongated crystals that are thicker than needles (for more on crystal shapes and habits see Geology.com and Wikipedia). So I initially thought the small long features in Stone 3 could be crystals, maybe small prismatic ones. However, when I posted photos of the stone on the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”, it was suggested that these were not crystals but fossils, maybe sponge spicules. I have just done some research on spicules and am not convinced that is what we can see in this stone. Most spicules have a more complex character, and the rare long thin ones are said to come to a point. However, I have realised that another stone in the Calendar, August’s Stone 3, could contain fossil sponge spicules. So I will go into more detail about them when dealing with that stone in the next Post. In the meantime, May’s Stone 3 (above) remains a mystery. NOTE: 12 November 2024 – John Taylor, a rockhound and fossil enthusiast who lives in Scotland, visited me today and we discussed this stone. John looked closely at it under a magnifying glass and with a blacklight torch. The latter revealed some calcite present, and John thought the stone was most likely a shell fossil, probably of an oyster of some kind. The long thin features that had caught my eye are part of the shell material remaining after some erosion. Thanks, John!

My guess is that Stone 4 (below) is a brecciated flint. I am not confident in this identification as I don’t often visit Ward Beach, only once or twice each trip, and I am not yet very familiar with many of its stones. However it is clear that tectonic stress has partly crumbled this rock, giving rise to small fragments and opening up veins. It appears the veins are filled with a translucent mineral as you can see the brecciated material through them. In one vein, the internal surface of the grey material seems to have some white mineral over it, as a sheet-like coating.

Flint is a sedimentary cryptocrystalline form of quartz, closely related to jasper and chert. In other words, quartz has been put under pressure and heat to the extent that the crystals become too small to be recognised and separately distinguished even under an ordinary microscope. “New Zealand Geology, An Illustrated Guide” by Peter Ballance (2017), an excellent e-book freely available online, has a map of the Kaikoura Ranges on page 247. The map shows the distribution of “Amuri limestone and associated flints” aged between 25 and 65 million years. There is a section of this limestone at the coast just near Ward Beach (just south of Chancet Rocks on the map, below centre).

Ballance writes on page 248: “Cover strata in Marlborough include a prominent fine-grained white limestone (really a hard chalk) called the Amuri Limestone. It originated as a deep-sea calcareous ooze on the continental slope of the time, 65–25 myr ago, during phase three of New Zealand’s formation. Like its English equivalent in the White Cliffs of Dover, it contains nodules of dark grey chert or flint, made of fine-grained silica.” In a couple of past visits to the south of England, I got used to seeing flint on beaches there, such as at Slapton Sands. I even have a piece of flint I found near Grime’s Graves, a Neolithic flint mining complex in Norfolk. 

JUNE – SEADOWN BEACH, KAKANUI

June’s beach scene of Seadown Beach (below, left) was photographed on 5 September 2024, on my way south. I had two fossicks that day and the photo was taken when I first arrived there, just before 10 in the morning when the tide was low.

The first stone for the month of June was found on Seadown Beach on 23 March, during a two hour fossick on a foggy morning.

The stone is most likely a quartzite which is common on this beach. However, what is most interesting about the stone are the veins in it. Not only are they in an interesting arrangement, dividing the stone into three sections, but they appear to have chalcedony (agate) in them. This is suggested by the thin white traces that become clear in the close-up images.

June’s Stone 2 is an interesting mish-mash of bits and pieces. It is a fossilised piece of ancient seafloor which I found in the afternoon of 30 September, a day on which I had two fossicks on Seadown Beach.

The two larger spherical features within the stone are fossil rhodoliths. Rhodoliths are marine nodules of algae made up mainly of calcium carbonate, and they can accumulate on the sea floor to resemble beds of coral. My research has shown that fossil rhodoliths are known to be present in the North Otago region. See TumbleStone Blog’s “Fossil Rhodoliths and Farewell to Kakanui’s Seadown Beach” for more information. The stone will be relatively soft, not unlike limestone, and so won’t polish in a tumbler.

Seadown Beach has a wide range of interesting jaspers (for example, see the four at the beginning and the one at the end of this Post and the nine in the middle of this Post). Stone 3 for June is a dark red jasper, though it lacks the hematite that would intensify its colour. I found it on the morning of 30 September.

This jasper has been brecciated by tectonic stress, the forces of the earth’s movement. Then it got into hot water. This hot water flowed through the gaps and fragments and veins and left behind the mineral silica. The silica was in a dissolved form in the hot water then precipitated out when it cooled.

I found Stone 4 during the same fossick that produced Stone 2, in the afternoon of 30 September. That day I wrote: “The most unusual stone stone I found this afternoon was this small dotted one, a type I have not seen before.”

It is a similar colour to a number of types of stones I have found on Seadown Beach, including quartzites, but I have not seen this pattern of dots in a stone from here before. I can compare it to Gemstone Beach’s poppy jaspers (see July Stone 4) and spotted argillites (see Stones Gn21, 23 & 27 here), but it is not one of those. Another mystery, for the present.

The months of July (Slope Point) and August (Gemstone Beach) in the 2025 TumbleStone Calendar are featured here. These 2025 Calendars are for sale, with postage within New Zealand included, for $30 non-rural, $35 rural. Contact me at john.tumblestone@gmail.com.