TS2 – GEMSTONE BEACH AND ITS STONES: AN INTRODUCTION FOR THE PASSING MOTORIST – PART FIVE, TRANSLUCENT STONES

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. Part One presented the location and geographical context for Gemstone Beach, and described its carpark and local beach warnings. Part Two set out the main features of Gemstone Beach and noted different beach conditions. Part Three provided a general introduction to the stones of the beach, referring to some already existing sources of information, and discussed the meaning of “gemstone”.  Part Four provided some brief definitions of important geological terms useful in later Posts. This Post is the first of a number on different stones listed by colour.

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 often open to correction. There is an amazing diversity of stones on that beach, each unique in some way. In these Posts, I am sharing what I know – it is not expert knowledge and it is not comprehensive. 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. I have also included photos of stones that I have found up to about 400 metres from Gemstone Beach, along the coast of Te Waewae Bay – the same types of stones can be found for many kilometres along this coast.

After some thought, I have decided to organise my presentation of the stones of Gemstone Beach by COLOUR. The main advantage of this is that colour is probably the most obvious characteristic of a stone. However, colour perception can vary from person to person, and I am aware that I don’t always see subtleties of colour the same as others. Keeping this in mind, I will do the best I can. Also note that the angle of your phone or computer screen, and any “eye comfort” filter you have on your phone, will also affect the hue of the colour you see.

Remember, look at the stones when they are wet to see their colours clearly. Also, stones can have a lot of detail in them which can be missed at first glance. Look at a stone up close, carefully, turning it over to view all sides. Sometimes, taking a photo with your phone then zooming into the image can be quite revealing.

A NOTE ON THE PHOTOS OF STONES BELOW

Each stone is numbered – “T1” means Translucent stone #1. In all cases, the first photo is of the stone in my hand, giving an idea of its size and what it looks like from a slight distance. The second photo is a close-up one, revealing more details. The third photo is usually the stone held up to the sky, and a fourth photo is sometimes provided of the stone with a torch behind it – the third and fourth photos demonstrate its translucent character.

TRANSLUCENT STONES

Occasionally, a “translucent” stone can be found on Gemstone Beach. As with Stone T1 (see photos above), a quartz stone with some mineral inclusions, when this type of stone is held up to the sky or when a torch is placed behind it, some (maybe only a little) light passes through it. It’s like looking through dull or fuzzy glass. “Translucent” is one of the three main ways of classifying how light passes through a stone. The first type is “transparent”, meaning light passes through the stone easily, as through clear glass or a clear quartz crystal; “translucent” means only limited light is able to pass through the stone, so that an object held behind it would look fuzzy; and the third category is “opaque”, meaning light does not pass through the stone at all (most stones are opaque). An interesting web page on the three categories provides the following useful illustrations:

Make sure if you lift up a stone to see if light passes through it that you don’t end up looking directly at the sun! Using the torch on your phone can be a safer option.

This Post looks at four types of stones, or parts of stones, that can be found on Gemstone Beach that are translucent. Three of them are relatively rare while the fourth type is more common. There could be other types of translucent stones but these are the four I am currently aware of.

1 – Translucent Quartz Stones

The following are seven more quartz stones that are translucent to varying degrees. You can mainly see this when you hold the stones up to a light source. When doing so, you will also notice any other material in the stone that blocks the light – often this will be a mineral like iron oxide. Sometimes the mineral will be in an obvious opaque patch as in Stone T2, or in a tracery of fine threads only visible against the light as in Stone T3, or in a more noticeable pattern even before being held up to the light as in Stone T6. Stones T7 and T8 contain tiny black specks, again a mineral inclusion in the stones. Some of the stones do not let much light through at all, though it is possible to see a little way beyond the surface, into the depth of the stone. Maybe the phrase “partly translucent” might be a more accurate term to describe these ones.

Quartz stones are more often an opaque white, being milky in appearance (see the Post on white stones) but this translucent variety can also be found on Gemstone Beach.

Quartz is a form of silica and is one of the most common minerals on Earth. It is a component of a wide variety of sedimentary, metamorphic and igneous rocks. The white veins and bands seen in stones are usually composed of quartz. Its purest variety is a clear colourless form known as rock crystal. A violet coloured variety is amethyst and a pink form is rose quartz – there are many more variations. Quartz is usually viewed as an igneous rock that crystallizes from magma or precipitates around hydrothermal vents. The metamorphic rock that forms with a very high quartz content is called quartzite and some of the beach stones we call quartz could in fact be quartzite. For more on quartz as a mineral, see “Quartz – Te Ara Encyclopedia of New Zealand”, “Quartz” in University of Auckland Geology website, “Quartz” in Mindat.org, and “Quartz” (page 10) in Thornton’s “Gemstones”.

2 – Translucent Hydrogrossular Garnets

Some translucent stones on Gemstone Beach look initially like quartz but have a different feel and a slightly different appearance. They feel waxy, and they have a slightly duller look, although when wet they shine brightly on the beach. These are metamorphic stones with the mineral hydrogrossular garnet in them (they come from a rock called rodingite which has other material mixed in as well). Stone T9 (below) is a good example of their waxy and slightly dull character. Also, when these stones are wet, water tends to “bead off” their waxy surface, forming beads or droplets that roll off. The photos of Stones T10, T11 and T12 below show the variable translucent character of the stones. Stone T10 has some brighter white patches as well as three darker inclusions. Half of Stone T11 has a light yellow hue, probably from iron oxide. Stone T12 is translucent mainly at one end.

The following photos of other translucent hydrogrossular garnets from Gemstone Beach include images with a torch behind them, adding to the appreciation of the opaque areas that can also exist within these stones. Stone T13 has a tiny bead of water on it due to its waxy surface. Stone T14 has a small dark crystal in it – Thornton suggests this could be the mineral diopside (see page 9 of “Gemstones”).

Stone T18 (below) is a more glassy type of hydrogrossular garnet – this type typically has a green hue and sometimes contains small concentrated green mineral patches (chromite?). NOTE: March 2025: I have had a geologist identify this type of stone as chromite in quartz (not hydrogrossular garnet). When held up to the sky, no light seems to pass through the stone but when viewed with a torch behind it, a small degree of translucency is apparent.

In general, stones with hydrogrossular garnet in them are not common on Gemstone Beach but they can be found there. I usually find one or two each visit, and often find maybe a handful more by walking a few hundred metres further northwards along the Te Waewae Bay coast. However, some visits do not yield any specimens.

Hydrogrossular garnet is unlike other types of garnet, such as the more common dark red crystals used in jewelry. Chemically, hydrogrossular is a type of calcium aluminium garnet, with hydroxide partially replacing what is silica in other garnets. It forms as dense masses, not nicely shaped crystals. Hydrogrossular garnet was first identified by New Zealander Colin Hutton in 1943, from stones found in the Nelson area. Gemstone Beach and its nearby Te Waewae Bay beaches are well known for hydrogrossular garnets, which most likely come from the mountains of Fiordland, brought down to the sea by the Waiau River and its tributaries. Some rockhounds specifically go to Gemstone Beach to look for hydrogrossulars. Hydrogrossular garnet often forms as part of a rock called rodingite, which also contains the minerals plagioclase and actinolite. Rodingite starts off as a plutonic, volcanic or sedimentary rock that is then saturated with hot fluids that deposit calcium. On page 122 of the book, “A Photographic Guide to Rocks and Minerals of New Zealand”, it is noted that the Gemstone Beach rodingites formed from igneous rocks. I have written a short introduction to these stones in my TumbleStone Blog here, with a longer Post plus many photographs in my TumbleStone Two website here. There are only short entries on hydrogrossular garnet in Wikipedia and Mindat.org. There are also only brief mentions in “Te Ara Encyclopedia of New Zealand” and on page 9 in Thornton’s “Gemstones”.

3 – Mix including Translucent Quartz

Some stones that you find on Gemstone Beach look as if they consist of a mix of two or three main materials, one of which is transparent quartz. Here are three examples:

Stone T19 (above) seems to have small white crystals embedded in the translucent quartz while Stone T20 contains a mix of green material (maybe the mineral epidote?) and quartz. Stone T21 has a more complex mix of green with translucent as well as milky-white quartz. In these cases, the translucence provides a sense of depth, allowing you to see a little way into parts of the stone. If you hold the stone up to a light source, you are unlikely to see through the stone because of the presence of a lot of opaque material as well. This is why only three photos of each stone are being provided in this Section and the next, showing the stone as held in the hand then in more detail.

Photos of five more of this type of stone are given below, showing a range of different patterns and colours:

Other colours are possible in such mixed-material stones. It depends on the minerals that are present along with the translucent quartz. Often milky white quartz will also be seen there, as with Stone T26 above.

In general, this variety of translucent stone is not common on Gemstone Beach but the occasional example can be found. It usually takes close examination to identify them.

4 – Translucent Quartz Patches, Veins and Crystals

Some stones contain small patches or veins or crystals of translucent quartz. This type of stone is more common on Gemstone Beach than the three other translucent types, mainly because the features can occur in a wide variety of stones, including quartzites, jaspers, mudstones, and so on. You can’t identify the translucent character of the patches, veins and crystals by holding the stones up to a light source, as the rest of the stone material is usually opaque and doesn’t allow the light through. It is the fact that you feel you can “see into” these areas that lets you know they are translucent.

The following are three Gemstone Beach stones with patches of translucent quartz. In some ways, this type overlaps with the third type of translucent stone, “Mix including Translucent Quartz”, the main difference being that this type has more localised translucent areas. The patches can be of irregular shape and could sometimes be viewed as an extensions of translucent veins of quartz. Indeed, many of these stones also contain thin veins.

Much more common than stones with translucent quartz patches are stones with translucent quartz veins. Stone T30 (below) is a green quartzite, Stone T31 is probably a lighter coloured quartzite, and Stone T32 is a red jasper – they all contain thin translucent veins, probably (I guess) introduced as layers of quartz laid down as hot water moved through cracks after the process of metamorphism. In some of the translucent veins, as with Stone T31, you can see tiny bits of the fragmented opaque stone material (sometimes referred to as “brecciated”, meaning broken up).

Below are photos of six more stones that contain translucent quartz veins. A range of differently coloured stones are involved, with many of them probably being quartzites. Note that Stones T33 and T37 also contain some milky white quartz.

Finally, you can find stones on Gemstone Beach that contain small translucent crystals of quartz. Here are three examples:

Such small translucent crystals can be found found in stones of other colours as well, most of the stones being quartzites.

The next Post in this Series is on stones that are predominantly white.

Go to GEMSTONE BEACH AND ITS STONES: AN INTRODUCTION FOR THE PASSING MOTORIST – PART ONE, LOCATION AND CARPARK

Return to GEMSTONE BEACH

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