The 2025 TumbleStone Calendar features beach stones collected during my two South Island fossicking trips in 2024. Details about the stones shown in the previous months can be found here: January and February (Gemstone Beach and Slope Point), March and April (Kakanui and Gemstone Beach), May and June (Ward Beach and Kakanui), July and August (Slope Point and Gemstone Beach), and September and October (Gemstone Beach and Ward Beach). November’s stones come from Seadown Beach, Kakanui, and December features more stones from Gemstone Beach. All of the stones in the Calendar were photographed in the rough while wet, held in my hand in bright sunlight.
NOVEMBER – SEADOWN BEACH, KAKANUI
The beaches on the North Otago coast between Oamaru and the mouth of the Waianakarua River are often stony, though the amount of stones and the degree of their smoothness vary considerably. On page 116 in her book, “The New Zealand Rockhound” (1981), Natalie Fernandez lists three places here as good for hunting interesting stones: All Day Bay, Kakanui River, and Waianakarua River. Seadown Beach lies a few kilometres north of these locations. It was in June 2020 that I first explored this coastline and stumbled across this beach, two kilometres north of the village of Kakanui. I found a number of gorgeous quartzites and jaspers there, and it quickly became one of my favourite fossicking sites. I named it Seadown Beach as Seadown Road intersects the main coast road here. November’s beach scene of Seadown Beach (below, left) was photographed on 30 September 2024, on my way home from my final fossicking trip this year.
I had two fossicks that day and the beach photo was taken at 3.30pm when it was approaching high tide. Stones 3 and 4 for the month were found that same day, but in the morning. I found Stone 1 the next day, on 1 October. It is what is known as a plasma jasper stone.
Plasma jasper has dark green within it – see The Quartz Page. Encyclopaedia Britanica notes: “Its colour, various shades of green, is due to disseminated silicate particles of different kinds – e.g., amphibole or chlorite.” I was not aware of this kind of stone before and initially thought it might have been limonite prase which can be found along the North Otago coast. However, a member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” suggested the ID of plasma jasper to me recently.
Stone 2 is a beach agate, a chalcedony stone, a cryptocrystalline form of silica (see Wikipedia on chalcedony). I picked it up on Seadown Beach on 5 September. Here it is, held up to the sunlight.
Beach agates are often found on the North Otago and Canterbury coast. Chalcedony is usually translucent, allowing some light to pass through it. However, the presence of minerals within it can block the light and give rise to interesting shapes. The following is Stone 2 without the light behind it:
Sometimes the term “agate” is reserved for chalcedony with bands inside it. The presence of significant amounts of opaque minerals within chalcedony results in stones like jasper – see Rocktumbler.com.
Stone 3 is a specimen of a type of stone I first found on Seadown Beach in June 2022. I picked it up on 30 September, and it contains lots of tiny bryozoan fossils.
Bryozoans are one-celled animals which live in colonies. There are about 1,500 living species. Most marine bryozoans have a partially calcified, hard body wall, which is subject to the process of fossilisation. There are many different types of bryozoa, some shaped like branching twigs, some like fans, some lacy. 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. More details are available in this Post on the Seadown stones.
Stone 4 is an “unknown” stone. It kind of looks like a mudstone but has light-coloured orb-like features in it that I have not seen in mudstone before.
Sometimes, I know, such orbs result from the clustering of light-coloured minerals during rock formation. I have found maybe three of these stones on Seadown Beach, this one being picked up on 30 September.
DECEMBER – GEMSTONE BEACH
The busiest part of Gemstone Beach is always the 100 to 200 metres in front of the carpark. December’s beach scene captures that area just after noon on a Friday on 23 February. There are 21 people and two dogs present.
December’s Stone 1 is a jasper, a variety of opaque microcrystalline quartz. It was found, along with Stones 2 and 3, during a three hour fossick on a cold morning, on 8 September. The light-coloured flashes caught my eye on the beach.
I started tumble-polishing this stone yesterday.
Stone 2 is a grey banded argillite. Sediments of light and dark mud have been captured in the stone.
This is quite a large stone at nine centimetres in length. It is too big to tumble-polish so I have given it a home in the rock garden beside our garage which has an attached AirBnB apartment called “The Stones”. It has rained today so I took some photos of it.
Even when dry, the distinctive bands of Stone 2 can easily be seen. Brown banded argillite (see here) and green banded argillite (see Stones Gn8 to Gn13 in this Post) can also be found on Gemstone Beach. For examples of banded argillite from Montana, USA, see West Valley Naturalists – this shows that the banding on many of the Gemstone Beach stones is by comparison clear and well-defined.
Stone 3 is another jasper but its character is different from that of Stone 1.
The green in it is likely due to epidote. Jasper comes in a wide range of varieties.
Stone 4 caught my eye on the beach because of the darker mineral that lies in-between the lighter coloured fragments. Maybe this is a breccia.
The close-ups reveal the many tiny crystals that make up the stone, maybe of quartz. My guess is that the greenish-grey bands between the light coloured fragments could be epidote. However, this is a very unusual stone in my experience and I am unsure what it is.
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.
Stone number 4 is my favourite (bet you already knew that!) Those orbs and that colour – just gorgeous! I’m guessing a type of sedimentary stone?