A Blog About Stone Gathering, Tumbling and Polishing, and Rocks and Landscapes, from New Zealand – With Musical Interludes (john.tumblestone@gmail.com)
The final in this Series, this stone contains the same green-hued mineral as Stone #11. It’s most likely epidote, in quartz. In Stone #11, the green is in patches; in Stone #30, there are some striking thin veins circling it, which caught my eye when I spotted the stone on Gemstone Beach.
Stone #30, Side A.
Stone #30, Side B.
The stone is more spherical, less flat, than most of the others in this Series. There is a tiny hole on Side A and a thin scratch on Side B – maybe incurred during the tumbling process. Generally, a quartz stone like this one will tumble-polish very well.
Go to Stone #1 in the Stone of the Day Series for November.Go to the Series Index.
Many beach stones in New Zealand are dark in colour, with the main exception of white quartz. Lighter coloured stones stand out and draw the fossicker’s eye on the beach. This stone is an example of a lighter coloured one, with a white quartz vein in it. The close-ups reveal a range of light coloured minerals in it, along with some darker red patches of iron oxide.
Stone #29, Side A.
Stone #29, Side B.
Tumble polishing can be an inexact art. You cannot always obtain a perfect smooth polish. Close-ups often reveal tiny blemishes on a stone that are not picked up by the unaided eye. Stone #29 has a couple of tiny scratches in it, and some small bits have not taken a good polish. A return to a 400 grit tumble followed by a 600 grit stage then polish again might improve it.
About a kilometre north of Gemstone Beach. Te Waewae Bay coast.
Another tumble-polished black and white stone from Gemstone Beach – the other black and white ones in this Series are #2, #12 and #13. Most similar to #12, it contains some dark gray pieces not in that stone.
Stone #28, Side A.
Stone #28, Side B.
Despite being tumbled in a fine 600 silicon carbide grit for at least a week before going into the polish tumble stage, the stone has a number of small rough areas. At least some of these are likely to be due to some softer minerals being present. I am often surprised to see such rough areas when looking at the close-up photos – they are not easy to pick up by eye when inspecting the stones between stages.
Among the many green stones on Gemstone Beach are ones with layers of dark and light hues, like this one.
Stone #26, Side A.
Stone #26, Side B.
This is probably argillite, a mudstone – another green one can be found in this Post. The layers make up a series of bands in the stone – it is a variety of what is known as “banded argillite”.
Gemstone Beach banded argillites also come in gray (see Stone 6 in this Post) and brown (as in this Post and this Post). Some of them seem to have come under more heat and pressure than others, with their bands appearing smoother and “melted” at the edges – the second stone in this Post is a good example.
Stone of the Day #25 is a small gray stone, with some brown in it, but it was its thin light coloured bands that caught my eye when I saw it on Gemstone Beach.
Stone #25, Side A.
Stone #25, Side B.
Like Stone #22, it is quite small and plays an important role in tumble polishing, helping the process to be more efficient, contributing to the polishing of larger stones.
I have come across a small number of stones on Gemstone Beach like this one – their patches of warm green and warm orange catch my eye, along with what seems to be some brecciation creating “islands” of the colours.
Stone #24, Side A.
Stone #24, Side B.
The tumble polishing is not perfect but it does help to clarify the colours.
This stone has a lot of detail going on within it. It appears highly fragmented, with a hint of white quartz.
Stone #23, Side A.
Stone #23, Side B.
A member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” suggested that perhaps it is a brecciated jasper – a stone made up of small fragments of jasper cemented together in a fine-grained matrix of some sort.
I found this stone on Gemstone Beach on 11 February – it is the second stone featured in the Post “Southern Sojourn 2023(13): ‘Big Wave, My Friend’”. Photos of the stone when found on the beach are below. I was intrigued by the cross-shape white quartz veins on Side B – but the tumbling removed a lot of that.
Another interesting stone found on Gemstone Beach earlier this year! This one is quite small, the smallest of the Stones of the Day so far. It is 2.5 cm wide and 1.5 cm high. Such small stones are necessary for efficient tumble polishing as they increase the surface area when the stones tumble against each other in the rotating barrel – that way, larger stones have more of their surfaces hit by smaller stones.
Stone #22, Side A.
Stone #22, Side B.
This is a reasonably common type of stone along the south coast. I have found many of them at Gemstone Beach and also Riverton Aparima. I was initially told by someone that it was a “Slope Point rhyolite”. It is certainly a form of rhyolite (or very close to it), and similar stones can be found at Slope Point, but the distance is too far (and too westerly) for it to have come from there. I sent five photos of a number of these stones in June 2019 to the geologist Nick Mortimer, asking if he could assist in identifying them more clearly. These are the photos I sent:
Image a1.
Image a2.
Image a3.
Image a4.
Image a5.
Dr Mortimer kindly replied in a very informative manner. He wrote: First of all, they are fundamentally igneous rocks because of the porphyritic texture – the well-shaped feldspar phenocrysts in a finer grained matrix. Second, I agree it they could be rhyolitic or, because of lack of visible quartz, more likely dacitic… Third, the pale streaks are interesting. Some siliceous lava flows can have a flow foliation like that. But I think it just as likely that the rocks are dacitic ignimbrites (welded tuffs) rather than pieces of lava flow. The a2 image has a reddish lithic clast that would support this. Fourth, the apple green colour (especially replacing feldspar in a2) is the metamorphic mineral epidote. This points to them being quite old geologically, as you might expect for pebbles on the Southland coast. My guess is that they were eroded from areas of the latest Jurassic/earliest Cretaceous Loch Burn Formation of eastern Fiordland. Equivalents run in a belt from Nelson to Stewart Island and are called Drumduan Terrane. With all those hydrogrossulars you have collected, you’re probably aware that most of the Gemstone Beach rocks have come down the Waiau River and have sources in eastern Fiordland.
Note that it is suggested that these are more “dacitic” than “rhyolitic”. Dacite is a felsic extrusive rock, intermediate in composition between andesite and rhyolite (University of Auckland Geology). It contains less quartz than rhyolite. See Stone of the Day #4 for more information on dacite. My first significant Post on these stones, including reference to part of Dr Mortimer’s email, was in March 2020, during lockdown – see “Stay-at-Home Day Six”. I also noted there that New Zealand geologist Patrick Marshall originally came up with the term “ignimbrite” in the 1930s meaning “rain of fiery rock dust” (from the Latin “igni” for fire and “imbri” for rain).
I have just recently seen reference to a Danish website on ignimbrite. A friend on Facebook had it mentioned to her. The website is “Svens StrandstensSite”, which translate.com says means “Sven’s Beach Pebble Site”. The url is “www.rapakivi.dk” – “rapakivi” is a type of granite (see Sandatlas). One of Sven’s pages on ignimbrite – see here (translation in images below) – includes a discussion of different types, accompanied by photos. I was pleased to see some similarities to Stone of the Day #22. Below are screen shots of parts of Sven’s page, most of them in English. When you right click on the original page, you can opt to “Translate to English” – the resulting translation has some weaknesses but much of the geological material seems to be quite reasonable.
English version, using Google Translator by right clicking on original page and choosing “Translate to English”.
Continuation of top of page.
Classification of ignimbrites, part one.
Classification of ignimbrites, part two.
Classification of ignimbrites, part three.
Photos of ignimbrites.
Photos of ignimbrites.
Photos of ignimbrites.
The two close-ups at bottom left above are very similar to Stone of the Day #22.
NOTE, 10 September 2025: Some discussion has arisen on the Facebook Group“New Zealand Lapidary, Rocks, Minerals, Fossils”about the identification of this stone. A few people suggest it is not ignimbrite, partly because it is too hard, but is rhyolite. Leaving aside the issue that ignimbrite is usually a form of rhyolite, I undertook some more research. Ignimbrite tends to be a poorly sorted relatively soft rock – unlike this stone. However, at one end of the spectrum, ignimbrite can be “welded” which consolidates a hard glassy appearance, and one source says it can be up to 6 on the 10-point Mohs scale of hardness. Furthermore, “darker-coloured ignimbrites may be densely welded volcanic glass” (Wikipedia). One Facebook comment suggested that the stone is more likely to be “a metamorphosed banded rhyolite”, which seems to me to be a good possibility too. As Wikipedia notes, metamorphism is the transformation of a rock which changes its mineral composition or texture. “Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation.” In general, it is always hard to deal with conflicting evidence for an identification and an open mind, considering the range of possibilities, is important.
I always pick up spotty or speckled stones because they usually have interesting detail, revealed especially by close-up photos. This stone is a good example.
Stone #21, Side A.
Stone #21, Side B.
Stone of the Day #21 looks to me like an amygdaloidal stone. An “amygdale” (also known as an “amygdule”) is an infilled vesicle in an igneous stone – a vesicle is a tiny hole originally formed by a gas bubble in the cooled rock.These holes then fill with heated 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 term “amygdale” comes from the Latin for almond, reflecting the almond-shape of many such vesicles, although some amygdales can be very circular.
