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 learned so much from this today. I’ve added the info to my identified stones album on flickr so I don’t forget! I need to change some of those stones in my collection that I’ve labelled rhyolite.