This Series of Posts provides an introduction to Gemstone Beach (Orepuki, Southland, New Zealand) and a basic guide to many of the stones commonly found on that beach. Previous Posts = Part One: Location and Carpark; Part Two: Main Features of the Beach; Part Three: Sources of Information & What is a Gemstone; Part Four: Brief Introduction to Geological Terms for Stones; Part Five: Translucent Stones; Part Six-A: Stones that are Predominantly White; Part Six-B: Stones with White Spots & Crystals; Part Six-C: Stones with Opaque White Veins & Bands; Part Six-D: Stones with Opaque White Patches.
DISCLAIMER: Over the past six years I have visited Gemstone Beach more than 150 times, also walking further along the Te Waewae Bay coast. But I am not a geologist – I am an amateur beach stone collector and polisher. I have done some reading and research about different types of stones, but my knowledge is limited and open to correction. Some of the photos used in these Posts were taken at Gemstone Beach in April 2023; some come from my fossicking trips there over the past three or four years; and a few were taken after I looked through my “finds” in my stone shed at home. All the stones are “rough”, not polished. The coverage of stones is not comprehensive and is biased by my own interests and what catches my attention. The photos are not perfect, often having some reflections in them. But I hope these Posts prove useful to the “passing motorist” and others who find themselves on this interesting beach.
Green-coloured stones are quite common on Gemstone Beach. Actual greenstone (pounamu) is not found there, though semi-nephrite is said to be present. This Post is the first of a number on Gemstone Beach’s green-coloured stones.
Green is a colour arising from the mix of two primary colours, blue and yellow. Many shades of green arise from the different proportions of blue and yellow. Keep in mind that colour perception varies from person to person and even from culture to culture – one website notes that the ancient Greeks used the same name for blue and green and considered them the same colour. Personally, when I look at some of the many many shades of green (300 claimed by this website, 134 claimed by this one), some of them look gray to me and some of them look more like blue (also see Wikipedia for shades of green). You may very well see the shades differently from me.
Furthermore, photos sometimes seem to fail to capture the exact hue of a stone as seen face-to-face. Keeping all this in mind, I will do my best to present a selection of Gemstone Beach’s green stones.
Why are some stones green? What minerals cause the green colour? One website lists such common green minerals as chlorite, actinolite, epidote, glauconite, olivine, prehnite, and serpentine. In “A Photographic Guide to Rocks and Minerals of New Zealand” (2011) by Nick Mortimer, Hamish Campbell and Margaret Low, there is a table on mineral identification that lists seven green minerals (see below, far left) which is very similar. I don’t know much about these and their relevance to Gemstone Beach stones, except for epidote. EPIDOTE is typically a pistachio-green silicate mineral, although it can also be a darker green. In New Zealand, it is widespread in veins in greywacke, schist and granite and is the main mineral in the iron-rich mudstones of Otago and Southland – such as argillite! Sandatlas has an excellent introduction to epidote, and “A Photographic Guide to Rocks and Minerals of New Zealand” (2011) by Nick Mortimer, Hamish Campbell and Margaret Low has an entry on it (see below).
Pumpellyite is also a green mineral, closely related to epidote. The Riverton Aparima Museum displays specimens (see photo above). According to Wikipedia, pumpellyite can occur in amygdaloidal stones. Mindat records that it can be found at Riverton’s Howells Point (Back Beach). I don’t have the expertise to identify pumpellyite in Gemstone Beach stones. At the end of this Post are comments on other green-causing minerals.
GREEN ARGILLITE is probably the most common stone of this colour on Gemstone Beach, and there are a number of significant variations of it, as we shall see. As noted at the end of Part 6-C in this Series, argillite is a hardened mudstone, consisting of much smaller grains than greywacke sandstone and feeling much smoother. In New Zealand, greywacke and argillite often exist side by side in layered bands or strata. And, like greywacke, argillite is sometimes deformed, fractured and veined because of significant amounts of tectonic movement over a long period of time (see University of Auckland). On Gemstone Beach, argillite stones come in gray, green and red, with some banded argillites being brown as well. A web page on argillite in part of the United States notes that argillite’s colour is the result of a variety of factors, including the nature of the sediments that make it up, the relative amounts of sediments present, and the chemistry of the sea water when the rock formed. For example, red argillite indicates the presence of iron in the rock, which oxidized when exposed to oxygen. Blue and green argillite indicate the absence of oxygen when the layers of sediment were compressed under the sea.
The first part of this Post looks at green hues of plain argillite stones. Banded argillite is then introduced (Stones Gn8 to Gn14) along with some examples of veined argillite (Stones Gn15 to Gn19). There follow sections on spotted argillite (Stones Gn20 to Gn33), argillite breccia (Stones Gn34 to Gn39), and finally trace fossils in argillite (Stones Gn40 to Gn62).
The most common green argillite stone on Gemstone Beach is a very pale green in colour, almost gray. In fact, the photos of this type of stone often don’t convey its green tinge. Stones Gn1, Gn2 and Gn3 illustrate at least part of the range of gray-green hues, with Stone Gn3 also having a brownish tinge.
Next are two examples of light gray-green argillite stones with darker bands and/or veins. The darker material likely started out as a different layer of sediment. Stone Gn5 contains veins of either coarser grains or brecciated material – such detail make a fascinating view.
Some argillite stones on Gemstone Beach are a darker green. To my eye, they are a blue-green hue. Two examples:
Argillite also comes in banded, veined and spotted variations. Specimens of banded and spotted argillite can be seen in the Riverton Aparima Museum (Te Hikoi) and are featured on the Museum’s poster of sample stones from Gemstone Beach. Firstly, the Museum’s banded argillites:
Often, a mix of light and dark greens can be seen in Gemstone Beach’s banded argillites. Layers or bands of differently hued sediments are preserved in these stones. Generally, banded argillites come in green, gray, or brown, sometimes with white bands, and there is variation on how “melted” the bands can appear (also see Stone W114 in Part Six-C, Stones with Opaque White Veins & Bands and the discussion following that).
Sometimes the banding of a stone can be a mix of mudstone (argillite) and sandstone. In sandstone, the grains are larger and can be seen by the eye, as in Stone Gn14:
Some argillites appear veined rather than banded. Here are five examples – note that Stone Gn17 also has some banding, and Stone Gn18 is the same stone as W88 in a previous Post in this Series, Part Six-C, Stones with Opaque White Veins & Bands. Except for Gn19, whose vein is dark in colour, the veins are likely to be a form of silica (e.g., quartz).
To turn now to spotted argillite. Specimens from the Riverton Aparima Museum:
A wide range of colours are involved in Gemstone Beach’s spotted argillites, and the size of the spots varies. Sometimes the spots are darker than the rest of the stone, sometimes lighter. Sometimes the spots are circular, sometimes more loosely defined. The following 12 stones are ones that I suspect to be spotted argillites, though I cannot be completely certain of their identification. They certainly illustrate an interesting range of “spots”.
Where do the spots come from? Some research suggests to me that they occur when argillite comes under increased pressure and heat, more than is usual for a sedimentary rock. A technical article mentions the idea that one of the first signs of argillite becoming metamorphosed (when it is put under increased pressure and heat) is the creation of spots when certain minerals become concentrated in specific areas. Continuation of metamorphism turns argillite into a metamorphic rock, such as slate or a type of schist.
There is a type of stone I have seen a number of times on Gemstone Beach or nearby that is also possibly a candidate for spotted argillite. The lighter coloured parts seem harder than the dark, often being slightly less worn down. Sometimes the pattern can be almost regular, like a turtle shell, as in Stone Gn32. Other times, the pattern is more irregular, as in Stone Gn33.
Another type of green argillite on Gemstone Beach is one of two of the many forms of breccia. A “breccia” stone consists of small angular fragments of one or more rocks cemented together in a fine-grained matrix. The small fragments come from some form of erosion and their angular shape means they have not “travelled” far, that is, they have not had the chance to be smoothed by rubbing against other fragments or stones. Sometimes, massive stresses on a rock tears fragments from it which are then cemented in a matrix – this is often known as tectonic breccia (see Stones Gn34, Gn35 and Gn36 below). Other times, sedimentary breccia occurs, when fragments from different stones end up cemented together in a fine-grained matrix (see Stones Gn37, Gn38 and Gn39 below). See this Post for some discussion of Gemstone Beach breccia.
The final section of this Page on Gemstone Beach’s green argillite considers trace fossils. These are quite common on Gemstone Beach, and come in a wide variety of colours and shapes. They can also be found elsewhere in New Zealand. Locally they tend to be called “fossil worm cast” stones (see Heritage Trails sign below), though in fact they are likely to be produced by a range of types of crawling and burrowing animals. A “trace” fossil is not a fossil of the animal itself but of something it has left behind. In the book, “A Photographic Guide to Fossils of New Zealand“(2013), it is pointed out that trace fossils “record an animal’s moving, exploring, escaping, hiding, breathing, hunting, feeding, excreting, reproducing, growing, playing, fighting, dying, or resting” (page 10).
It is said that fossils are found only in sedimentary rocks, like argillite. Plant and animal remains and traces don’t survive the forces that produce metamorphic rocks nor the extreme heat associated with igneous rocks. Unfortunately, the Riverton Aparima museum’s photographic poster of “Sample Stones” from Gemstone Beach does not actually show the traces in the “fossil worm castings” on the poster, though the museum’s display includes two good examples of “worm trace fossil” stones (see photos above).
The first three Gemstone Beach trace fossil green argillite stones illustrate something of the diversity of trace shapes. Sometimes a number of small shapes can be found in a stone, sometimes just one or two larger ones.
Sometimes the traces are lighter in colour than the host rock, as with Stone Gn43 below, and sometimes the traces are darker, as with Stone Gn44.
Trace fossils of different colours can be found in green argillite. The next three stones, for example, have red or brown traces.
The next six trace fossil stones show the variation in the clarity and nature of the trace shapes. Trace shapes have been given names – for example, four of these stones – Gn48, Gn50, Gn51 and Gn53 – are good examples of “protovirgularia”, a shape defined by a series of chevrons in a line (for more on this, see this Post).
Keep in mind that trace fossil shapes are three dimensional. This means that some of them may not lie lengthwise on the surface of a stone (as with nearly all the previous examples) but may run through the middle, a cross-section of the “tube” emerging from the stone. Below are four photos of three dimensional trace fossils, not smoothed off by the tumbling action of the sea. The first two photos are from Mokomoko Inlet near Bluff, some 80 kilometres west of Gemstone Beach. The first photo, from a geological reserve, is introduced like this: “Fossil worm casts are a feature of these rocks. The casts were left as worms tunneled through mud and compacted the sediment behind them. The mud solidified to rock and on exposure, the softer part of the rock eroded leaving the raised worm casts…Smoothed pebbles with the characteristic chevron pattern of the worm casts are common on Southland beaches” (page 104 of Lloyd Esler’s 2013 book, “Omaui and the New River Estuary”). The traces in the second photo are described as “faecal pellets secreted by worms – the relative compaction may be an expression of the extent to which the material has been digested” (page 669 of David Mossman and Lucy Force’s 1969 article, “Permian fossils from the Greenhills group, Bluff, Southland, New Zealand” in the “New Zealand Journal of Geology and Geophysics”).
The third photo (above) is from New York and the traces are said to be of “fossil worm burrows” (the original is here). The fourth photo is of burrows left by an “ancient crab” in rocks at Lyme Regis in England (see here).
Stone Gn54 (below) is an example of a number of round or small lineal traces, in the top part of the stone, which may be cross-section views of traces running through the middle of the stone.
Some green argillite stones, like Hn55 (below), contain what look like quite small traces – it can be difficult to know whether they are tiny lineal traces or cross-sections of a trace “emerging” from the stone.
Some of the trace fossil markings can be quite complex, containing different kinds of traces. Stone Gn56 is an excellent example.
Sometimes the traces can be rather plain, not having the sinuous chevrons of protovirgularia, though some of the traces in Stone Gn57 have edges with small markings hinting at chevron shapes.
The final five trace fossil stones below contain markings that might or might not be trace fossils. The markings are sometimes quite small, as in Stones Gn58 and Gn60, or blurred, as in Stone Gn59. In Stone Gn61, could the light-coloured shape have been caused by ripples in the sediment as a fish swam by? Or some other disturbance to the sediments? And are some of the lighter lines in Stone Gn62 caused by water currents stirring up the sediments? It is not always easy to determine whether markings are trace fossils or not.
Finally in this section, there are stones on Gemstone Beach which could have trace fossil markings or they could be a type of breccia. These can be found every now and again, and I have always been unsure about them. Stone Gn63 is a good example of this type of stone.
I tend towards the view that this is brecciated, that the lighter material is a sediment stratum that has been torn apart. However, I am far from certain about this.
The trace fossils on Gemstone Beach have always fascinated me. Not only green argillite can contain them, but other colours of argillite as well, such as red and gray. I have written a series of blog Posts on them in attempting to understand them, and at one point I put together a small photo book on them. The fourth inductee into the TumbleStone Hall of Fame is a trace fossil stone, and a recent Post on the stones is in a 2023 Series on a fossicking trip to the South Island.
BRIEF COMMENTS ON THE MAIN GREEN-CAUSING MINERALS (based mainly on information in the book, “A Photographic Guide to Rocks and Minerals of New Zealand” 2011 by Nick Mortimer, Hamish Campbell and Margaret Low): EPIDOTE and PUMPELLYITE have already been discussed above, near the beginning of this Post. OLIVINE is typically yellowish green to olive-green and commonly found in dark primary igneous rocks (such as basalt and gabbro). Dunite consists of over 90 per cent olivine. For more on olivine, see University of Auckland Geology and Geology.com. AUGITE is a medium dark-green colour and common in igneous rocks like basalt, andesite, gabbro and diorite. See University of Auckland Geology and Wikipedia. DIOPSIDE is augite without any iron in its crystals, and specimens are included in the Riverton Aparima Museum poster and display (see photos below). It can also be blue, even other colours (see Mindat). I don’t have the expertise to identify diopside in Gemstone Beach stones. ACTINOLITE is a pale green, often occurring with serpentine, chlorite and epidote. It tends to be fibrous in character, and is the main mineral in nephrite. See Minerals.net and Wikipedia. CHLORITE is a member of the mica family, usually a dark green colour. It typically occurs in metamorphic rocks and generally provides the green in greenschist. It also fills in the cavities in ancient basalt lavas, such as the Permian volcanic breccias near Riverton Aparima. See University of Auckland Geology and Geology.com. GLAUCONITE is also a dark green mica but occurring only in marine sedimentary rocks. See Wikipedia. SERPENTINE ranges from a pale green to a dark green. It tends to be soft and as a mass it has a smooth soapy feel. It is usually found as serpentinite in New Zealand. See Geology.com and Minerals.net.
The next Post in this Series (not yet written) looks at other kinds of green stones that can be found on Gemstone Beach.
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