This is a type of stone that can often be found on Gemstone Beach or nearby.
I am unsure what it is, but the white bits and the variations of dark red and purple make it interesting.
Go to Stone of the Day #28. Go to the Series Index.
Among the many green stones on Gemstone Beach are ones with layers of dark and light hues, like this one.
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.
Go to Stone of the Day #27. Go to the Series Index.
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.
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.
Go to Stone of the Day #26. Go to the Series Index.
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.
The tumble polishing is not perfect but it does help to clarify the colours.
The fifth stone in this Southern Sojourn Post is a similar stone.
Go to Stone of the Day #25. Go to the Series Index.
This stone has a lot of detail going on within it. It appears highly fragmented, with a hint of white quartz.
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.
Go to Stone of the Day #24. Go to the Series Index..
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.
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:
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.
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.
Go to Stone of the Day #23. Go to the Series Index.
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 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.
The main stone for December in the TumbleStone 2024 Calendar is a similar amygdaloidal one, also from Gemstone Beach. Another specimen can be found in this 2022 fossicking Post.
Go to Stone #1 in the Stone of the Day Series for November.
By the end of October 2023, I had finished tumble-polishing some of the stones I collected from Gemstone Beach during my “Southern Sojourn” at the start of the year. Throughout November, I posted one of these stones each day. Each Post provides images of a stone, including close-ups. Sometimes I am able to provide an identification, not always. Occasionally I also provide a little information about the type of stone.
Stone #1 Metamorphic with epidote (with additional brief comments on the mineral epidote)
Stone #2 Black and white
Stone #3 Quartz fragments/crystals and iron oxide
Stone #4 Light coloured igneous stone, maybe dacite? (with some information on dacite)
Stone #5 Thulite (with additional brief comments on the mineral thulite)
Stone #6 Quartzite
Stone #7 Unusual red hue (stone features in TumbleStone 2024 Calendar)
Stone #8 Hematite jasper (with additional comments on hematite)
Stone #9 Speckled dark green stone
Stone #10 Iron oxide dapples
Stone #11 White quartz and epidote green
Stone #12 Conglomerate or crystals? (with additional comments on conglomerates)
Stone #13 Black igneous stone with thousands of small white crystals
Stone #14 Brecciated stone
Stone #15 Rhyolite, perhaps ignimbrite (with brief comments on ignimbrite)
Stone #16 Light gray mudstone with darker markings (with comparisons with the rough stone)
Stone #17 Stone with bright purple veins
Stone #18 White quartzite
Stone #19 Patchy green stone
Stone #20 Rhyolite (with brief comments on rhyolite)
Stone #21 Amygdaloidal stone (with brief comments on amygdales)
Stone #22 Dacitic ignimbrite (with extended comments)
Stone #23 Fine-grained breccia
Stone #24 Warm green and orange patches
Stone #25 Small gray banded stone
Stone #26 Green banded argillite
Stone #27 Dark red, purple and white stone
Stone #28 Black and white stone
Stone #29 Quartz with light brown, pink and purple minerals
Stone #30 White quartz with green epidote
An igneous stone, most likely rhyolite, this stone was found on Gemstone Beach or the coast just north of there along to the Waimeamea River mouth. I initially learned to recognise rhyolite stones at Slope Point and then, afterwards, started to notice them at Gemstone Beach as well. Stone of the Day #20 is a type of rhyolite that I now see each time I fossick at Gemstone Beach (there’s a very similar stone near the end of this Southern Sojourn Post).
It is not always possible to get a good smooth polished rhyolite stone but this one turned out quite good. The small light-coloured crystals have interesting shapes.
Rhyolite is a volcanic “felsic” rock, having a high silica and feldspar content. (“felsic” comes from “fel” from feldspar and “sic” from silica). “Felsic” rocks are not only silica rich but also tend to have a lot of aluminium, sodium and potassium, often giving them a lighter colour. Rhyolite is an unusual mineral as it can take quite different forms – University of Auckland Geology notes that it can be pumice, ignimbrite, obsidian or flow-banded etc., depending on whether the molten rock has erupted or flowed, what its gaseous content is, and how quickly it has cooled. Geologyscience.com notes that rhyolite is the extrusive form of granite, but that rhyolite has a fine-grained texture with phenocrysts, which are small crystals sometimes embedded within the rock (as apparent in Stone #20).
Go to Stone of the Day #21.
Go to the beginning of this Series. Go to the Series Index.
There are a range of green hues in Stone of the Day #19, from a very light green to the pistachio green associated with epidote to a dark gray-green. A number of very fine clear silica veins are also apparent in the close-up images.
Its size is 4.5 cm long and 3 cm high. Parts of the stone are still a bit rough and there’s a large chunk that has been knocked out of the stone during the polish tumble, noticeable at the bottom left of Side A. The darker patches in particular have not taken a polish very well.
Go to Stone of the Day #20.
Go to the beginning of this Stone of the Day Series. Go to the Series Index.