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.
The previous Post looked at three types of stones, or parts of stones, that can be found on Gemstone Beach that are white: 1 – White Milky Quartz Stones; 2 – White Hydrogrossular Garnets; and 3 – White in Other Coloured Hydrogrossular Garnets. This Post looks at two other groupings of white stones: 4 – Stones with Opaque White Spots, and 5 – Stones with Opaque White Crystals. The next Post looks at stones with opaque white veins and bands, with the following Post looking at stones with opaque white “patches”. There could be other types of white stones on Gemstone Beach but these are the ones I am currently aware of that are most likely to be found.
4 – STONES WITH OPAQUE WHITE SPOTS
Some stones on Gemstone Beach are black (or maybe red, less often green) with white or light-coloured spots. These are usually either amygdaloidal stones (what the Riverton Aparima Museum poster describes as volcanic stones with “filled bubbles”) or they are volcanic stones within which crystals have formed as the rock has cooled (next section). It is not always easy to tell which are which. But many of these stones have a very interesting appearance and some are very attractive.
THE WHITE “SPOTS” OF AMYGDALOIDAL STONES
The following four stones with white spots are good examples of amygdaloidal stones. In general, the white “spots” are circular or deformed circles, with a well-defined boundary.
How are these amygdaloidal stones formed? Molten magma often contains dissolved gas which can form bubbles in the rock as the pressure is released on eruption. These bubbles can get trapped in the solidified rock, forming tiny holes (called “vesicles”). These holes then fill with 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 resultant “spots” are called “amygdales” (also called “amygdules”), a term that comes from the Latin and Greek words for almond, reflecting the almond-shape of many such in-filled vesicles. However, many amygdales are round and others can be irregular shapes, as demonstrated by the Gemstone Beach examples in this Post and the six specimens on display at the Riverton Aparima Museum (see photo below, far left). The three photos below to the right are of different kinds of amygdaloidal basalt.
“Amygdaloidal” is a term that refers to the way the stone has formed rather than the material it consists of. By far the most common volcanic amygdaloidal rock is basalt, though andesite, for example, can also be amygdaloidal. For more on amygdaloidal rocks, see Alex Strekeisen, Wikipedia, Ontario Beneath Our Feet, and University of Oxford Earth Sciences.
You can sometimes find quite attractive red amygdaloidal stones on Gemstone Beach with a range of amygdale shapes and colours, including white. While basalt is often a dark black or gray colour, red is also a not uncommon colour. Here are five examples which well illustrate the variations – note the irregular amygdale shapes in Stone W36 and the different colours in Stones W37 and W38:
Finally, four more examples of amygdaloidal stones found on Gemstone Beach, a couple of which have a noticeably green host rock. The book “A Photographic Guide to Rocks & Minerals of New Zealand” notes on page 58: “Geologically old basalts (older than 100 million years or so) have recrystallised and can be a dark green colour because they contain the minerals chlorite and epidote”. Also note that the colour of the amygdales in Stone W39 is not a bright white, almost verging on the translucent maybe.
5 – STONES WITH OPAQUE WHITE CRYSTALS
Most of the stones with opaque white crystals on Gemstone Beach are igneous in origin, as will be seen. To deal first with white and black stones made up of tiny crystals. A number of variations of these can be found on Gemstone Beach. The size of the crystals, their shape, and the ratio of white to black can all vary quite a bit, as illustrated by these seven examples which at first glance look nearly identical:
There is an amazing variety of white crystals in these stones. Some of the white crystals are quite small, some larger. They have different shapes, some being square or rectangular, some being elongated or stretched out. Some having an indistinct shape but many are individually apparent, while others appear clumped together.
These stones are all likely to be volcanic or plutonic stones within which crystals have formed as the rock has cooled (Part 4 in this Series briefly defines volcanic and plutonic as two types of igneous rocks, rocks formed by the cooling of molten material). I find it difficult to identify the type of rock each stone comes from, and the identity of the white crystals varies from quartz to feldspar, among others, but the process that produces the white and black crystal mix is clear (and will be described more fully below, after Stone W81). To the north and west of Gemstone Beach are geological features in Fiordland with origins in molten rocks. Due to glaciation and other forms of erosion, stones from these features have eventually made their way down to the Waiau River system and out to sea and then onto Gemstone Beach.
In the next seven white and black stones from Gemstone Beach, the white crystals are larger:
Sometimes with this type of white and black stone, the component of white can be tiny, as with Stones W57 and W58:
There is a wide variety of types of this dark-coloured stone with white crystals – it pays to look closely and carefully to check out the characteristics of the crystals. Three further examples of the variety of the nature and shape of the white crystals:
Sometimes the white component dominates a stone, with the white crystals seeming to clump together to make larger groupings:
Sometimes a dash of colour can be found in a stone of opaque white crystals, due to the presence of a mineral, such as the reddish-brown iron oxide in Stone W64:
Apart from Stone W64, so far the non-white component of these stones has been black or dark gray, with Stone W62 having the palest non-white matrix. But opaque white crystals can also be found in stones of other colours, such as these two green examples:
The green in stones on Gemstone Beach is often due to the presence of the mineral called epidote, although other minerals give rise to green as well.
Two examples of reddish-purple stones from Gemstone Beach that contain white crystals:
There is a variety of pink stones on Gemstone Beach, the pink caused by the presence of the mineral thulite, that often have white crystals showing within them. Some of these stones are even purple in colour, as with Stone W71. Thulite is often associated with quartz and, unlike most of the other stones with white crystals, is a metamorphic stone rather than an igneous one.
Five examples of brown stones with white crystals, the first four likely to be rhyolite:
There are also a variety of stones with white crystals on Gemstone Beach that are granite. Some are quite light coloured (known as “pale granite”) while others (known as “pink granite”) have a pink-orange feldspar in them:
Finally, to re-emphasise the great variation of stones on Gemstone Beach, a light coloured stone with a handful of opaque white crystals, or clumps of crystals:
What explains the different sizes and shapes of most if not all of the white crystals in Stones W43 to W81? The answer lies in their origin in molten rock. It all depends on how quickly the molten rock has cooled and hardened, and what minerals present in the molten rock make up the crystals. A “volcanic” rock cools on the earth’s surface and, because it has solidified quickly, the crystals have not had time to grow very large. But for a “plutonic” rock, the molten rock has cooled much more slowly underground, giving the crystals time to grow larger, sometimes as large as fingernails. Sometimes an igneous rock has both fine-grained and larger crystals – this is called a “porphyry”, referring to the “texture” of the rock. A porphyry has well-formed crystals visible to the naked eye (called “phenocrysts”), set in a very fine grained matrix (see the two photos below right). This happens when magma that has been slowly cooling and crystallising within the Earth’s crust is suddenly erupted at the surface, causing the remaining uncrystallised magma to cool rapidly (see “Textures of Igneous Rocks” in University of Auckland Geology). So a porphyry is intermediate between quickly solidifying fine-grained rocks and the slowly solidifying rocks with uniform larger crystals. Stones W54, W55 and W56 above are good examples of a porphyry. Note that “porphyry” is a term describing the relative crystal sizes in rocks, it’s not a rock type like basalt, rhyolite and granite.
There are a number of well-known shapes, or “habits”, of crystals, although the exact number is not agreed upon, and different terms are sometimes used for the same shape. The Mineralogical Society of America provides the following examples of shapes or habits (among others): blocky or equant (having equal sides), tabular (which can be thin or thick), prismatic, bladed (like grass or a knife), acicular (needle-like), and filiform or capillary (like hair or thread). There are also terms for different kinds of groupings of crystals, like drusy, radiating, dendritic and reticulated. See also Dexter Perkins and Geologyin.com for an introduction to habits. However, crystals in rocks tend not to grow freely in their typical shape, being crowded and squeezed by other crystals and material, or the conditions for their growth may have been inadequate or too short. It is therefore often very difficult to identify a rock type based on the crystal shapes in a stone, or the sizes of the crystals.
Despite these problems, it is worthwhile to consider some typical igneous rock types which may relate to our stones. Igneous rocks are categorised according to their composition and grain size, more than the type of crystals in them (see diagram below, at far left in next set of photos). For example, GABBRO is a plutonic rock consisting of about 50% to 90% of the dark minerals augite, olivine and hornblende. The pale mineral in gabbro is plagioclase feldspar. As shown in the diagram, gabbro does not contain any quartz. Gabbro has the same mineral composition as the volcanic rock BASALT but the grain size of the two rock types differs due to their different rates of cooling. The book “A Photographic Guide to Rocks & Minerals of New Zealand” notes on page 73 that gabbro can be found between Colac Bay and Pahia, a handful of kilometres away from Gemstone Beach. The Riverton Aparima Museum has a specimen of gabbro from Monkey Island (see photo below, centre), which is on the coast of TeWaewae Bay, just two kilometres south of Gemstone Beach.
DIORITE is a plutonic rock that is generally a little lighter in colour than gabbro, consisting of 25% to 50% of dark minerals (see photos above, right, which also illustrates the variation in diorite). A small amount of quartz may be found in diorite (as shown in the diagram above, far left). Fine-grained gabbro and diorite is sometimes called DOLERITE (or diabase), which tends to occur in dikes and sills (sheets of rock that are formed in a fracture of a pre-existing rock body when magma is forced through). The specimens of dolerite stones in the Riverton Aparima Museum, two of which are from Gemstone Beach, again show quite a variation in appearance (see photos below).
Stones W43 to W61 above all have white and black crystals. The ones with the smaller crystals are likely to be plutonic and the ones with larger crystals could be volcanic. However, I am unable to identify any particular stone as being gabbro or diorite, for example, or as GRANODIORITE, which comes between granite and diorite (it has more quartz than diorite). I don’t have the knowledge to do so. But these stones all started as molten rock and then their crystals formed as the rock hardened. It’s amazing to see how diverse is the mix of crystals, along with their shape and their arrangement.
Another plutonic rock mentioned in the diagram above (of mineral composition in igneous rocks) is GRANITE. It is the lightest in colour of the plutonic rocks, consisting of only 5% to 25% of dark minerals. The dark mineral is usually black mica biotite, sometimes hornblende. The pale minerals include white plagioclase feldspar, pink orthoclase feldspar and a pale gray quartz, with a white muscovite mica also possible. Granite has more quartz in it than the other rocks we have been considering in this part of this Post. The Riverton Aparima Museum has three specimens of granite (see photo below, second from left) showing how its colour can vary considerably. It also has on display bowls of pale and pink granite. The book “A Photographic Guide to Rocks & Minerals of New Zealand” notes on page 75 that granite is exposed along the coast between Orepuki and Colac Bay. More information about granite in general can be found here.
As noted above in relation to Stones W77 to W80, RHYOLITE stones can be found on Gemstone Beach. Rhyolite has the same chemical composition as granite but is a volcanic version of it.
Note: An excellent introduction to Fiordland’s geology, the most likely source of stones on Gemstone Beach, is the ten minute YouTube video “Fire, Ice, and Tremors – Episode 1: The Geologic History of Fiordland National Park”. Parts of this Post have drawn on Jocelyn Thornton’s (1985) book, “The Field Guide to New Zealand Geology”.
The next Post in this Series looks at a further two groupings of white stones that can be found on Gemstone Beach, stones with opaque white veins and opaque white bands.
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.
Return to GEMSTONE BEACH
HOME – FOSSICKING BEACHES – BEACH STONES – TUMBLE-POLISHING
24 thoughts on “TS2 – GEMSTONE BEACH AND ITS STONES: AN INTRODUCTION FOR THE PASSING MOTORIST – PART SIX-B, STONES WITH WHITE SPOTS & CRYSTALS”