TumbleStone Calendar 2025: March and April

The Introduction to this Calendar can be found here. The Calendar features beach stones collected during my two South Island fossicking trips in 2024. The stones shown in the months of January and February are described here. The month of March features Seadown Beach, Kakanui, which lies just south of the city of Oamaru in North Otago. April takes us back to Orepuki’s Gemstone Beach in Southland. Note that the stones in the Calendar were photographed in the rough and while wet, being held in my hand in bright sunlight. The photos often reveal more about the stones than I noticed on the beach. While none of eight stones below are labelled “unknown”, two of them have question marks after them. It is wise to take my identifications as those of an amateur tumble-polisher, I am not a geologist nor do I have the scientific means to confirm a stone’s character. My research and understanding are distinctly limited and ongoing.

MARCH – SEADOWN BEACH, KAKANUI

March’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 at 10 in the morning when the tide was low.

The first Seadown Beach stone for March is a quartzite. There are many yellow quartzites on this beach, some with a dash of red in them. But there are quite a few quartzites of pastel shades as well, mainly blue and grey. This find is maybe the best I have seen here of this latter type.

The stone was found on 30 September 2024, on my way back north. The TumbleStone Blog Post for that afternoon’s two-hour fossick was entitled “High Tide and A Few More Stones at Kakanui’s Seadown Beach”. Yellow, blue, grey and green quartzites are not uncommon on beaches in Otago and Canterbury, and I have found them, for example, further north at Birdlings Flat near Christchurch (see this 2016 Post, written before I knew much about quartzites). Quartzite is a term often used for a metamorphosed sandstone. In her booklet “Gemstones”, the geologist Jocelyn Thornton states on page 20: “Quartz sands cemented by silica make an extremely hard and durable quartzite which is found on some hillsides in Central Otago and washed down rivers and out along the coasts. These quartzites have cloudy patterns and take a very good polish.” More information on these stones can be found in the entry for Wednesday 26 May 2021 in this Post. Stone 3 below is also a quartzite.

March’s Stone 2 caught my eye because it is pink. If I saw it on Gemstone Beach rather than Seadown Beach, I would initially think it likely to be thulite (see April’s Stone 4 below). However, upon closely inspecting this stone, it looked and felt more like limestone, or something very like it. Limestone is well known in the North Otago area but it is usually whitish or cream-coloured.

As Te Ara The Encyclopedia of New Zealand states: “Limestone, a sedimentary rock abundant in New Zealand, consists mainly of the bones and shells of tiny marine fossils made of lime (calcium carbonate). Rocks with more than 50% calcium carbonate are considered to be limestone.” Also see University of Auckland Geology for basic information on limestone. March’s Stone 2 will be a relatively soft stone and won’t stand up to much tumbling – a light tumble in a fine grit will be all it needs to brighten it up a little. I found it on Seadown Beach on 6 September.

Stone 3 this month is another quartzite, but unlike Stone 1 it is a variety of the yellow ones on Seadown Beach. I found it the same day I found Stone 2.

The Blog Post from my fossick that day, 6 September 2024, features the wide range of Seadown Beach’s yellow quartzites. I wrote: “A type of stone that particularly stood out on the beach this morning was the yellow quartzite that can often be found here. I collected a few, always finding them hard to resist. Here are 20 of my quartzite finds… These specimens illustrate only part of the many variations of this type of stone. All of these have tiny quartz crystals in them.”

March’s Stone 4 is the most amazing stone I have found on Seadown Beach, just coming ahead of the bryozoan fossil stones from there (one of which features in the Calendar’s month of November). I found it on the afternoon of 2 September, during a 90 minute fossick. I was looking among stones right at the south end of the beach, in a tiny bay flanked by high rocks, when I spotted it.

It has been suggested by members of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” that it is agatised fossil bone. After some research, I have found a few photos on the internet of agatised dinosaur bone (see photos below) that have a lot of similarities – see, for example, Geologyin and Grafphoto.

I am not suggesting my Kakanui find is a dinosaur fossil but it definitely looks like agatised bone. A few days after writing this, on 7 November, I came across Thornton’s two specimens of fossil bone on page 25 of “Gemstones”. One of these has some resemblance to Stone 4, though Thornton’s photo is not very clear. She notes that some agatised fossil bones from Oaro (North Canterbury) are “from marine reptiles related to the dinosaurs and are about 70 million years old.”

APRIL – GEMSTONE BEACH

April is the second of five times that Gemstone Beach appears in the Calendar – it is the beach I visit most often. On 7 March, I spent five hours on the Te Waewae Bay coast, starting at Gemstone Beach. It was an especially windy day. Early on, I climbed a hill above the Taunoa Stream and took this photo (see below, left). It shows the eastern sweep of Te Waewae Bay with Monkey Island in the far distance and Pahia Hill in the background. The roof of the beach’s half-hidden house can only just be glimpsed centre left in the photo, on the other side of the Taunoa Stream. The two people on the beach in the middle distance are in front of the carpark.

The first stone that appears in April in the 2025 Calendar, at the top left of the page, is an amygdaloidal stone, sometimes known as a stone with “infilled volcanic bubbles”. I found it during a four hour fossick on 14 September.

I explain this type of stone as follows in the Post “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Six-B, Stones with White Spots & Crystals”: “Molten magma often contains dissolved gas which can form bubbles in the rock as the pressure is released on eruption. These bubbles can get trapped in the solidified rock, forming tiny holes (called “vesicles”). These holes then fill with mineral-rich fluids which leave behind deposits of minerals such as quartz, chalcedony (agate), calcite and zeolites – zeolites are a group of minerals with a crystalline structure made up of silicon, aluminum, and oxygen. The resultant ‘spots’ are called ‘amygdales’ (or ‘amygdules’), a term that comes from the Latin and Greek words for almond, reflecting the almond-shape of many such in-filled vesicles. However, many amygdales are round and others can be irregular shapes.”

April’s Stone 2 is the distinctive pistachio green of the mineral epidote. In New Zealand, epidote is widespread in veins in greywacke, schist and granite and is the main mineral in the iron-rich mudstones of Otago and Southland, including the argillite common on Gemstone Beach. See near the beginning of “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Seven-A, Green Argillite Stones” for more on epidote. In argillite, epidote is usually dispersed throughout the stone, giving it a uniform green colour – in Stone 2, it appears in small clumps, almost in an orbicular fashion.

I found this stone on 9 March. It is unusual for the intensity of the epidote in it.

Jasper can be found in many places in New Zealand. Along with chert and flint, it is a variety of opaque microcrystalline quartz. It can be a number of colours, dark red the most common but green and yellow also often found. There’s more information and specimens on pages 17 and 18 of Jocelyn Thornton’s “Gemstones”. Sometimes jasper can have a mossy character, like April’s Stone 3.

This stone also contains white quartz. I found it on Gemstone Beach on 7 March, the same day I took the beach scene photo for the month.

April’s Stone 4 is thulite, a pink stone. I found it on 8 September, the first fossick on Gemstone Beach for that trip, three hours on a cold Sunday morning, but a very productive time.

There are clouds of intense bright pink in this stone – some thulites are much lighter in colour, like this one found in June 2021. In my experience, many of the brighter stones don’t tumble-polish well, coming out rough and non-shiny, while the lighter coloured ones often come out much better. Thulite is the pink to reddish variety of the mineral zoisite. As  Minerals.net  reports, “The color of thulite is caused by the element manganese in its composition. Thulite often occurs associated with quartz, and is sometimes mottled with streaks or spots of white quartz.” Thulite is the national gemstone of Norway. You can read more about it on Wikipedia and Gem Adventurer.   

The months of May (Ward Beach) and June (Seadown Beach, Kakanui) in the 2025 TumbleStone Calendar are featured in the next Post. 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.