TS2 – GEMSTONE BEACH AND ITS STONES: AN INTRODUCTION FOR THE PASSING MOTORIST – PART SIX-A, STONES THAT ARE PREDOMINANTLY WHITE

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.

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.

I have organised 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, keep in mind that colour perception can vary at least a little from person to person. The angle of your phone or computer screen, and any “eye comfort” filter you have on your phone, can 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.

WHITE STONES ON GEMSTONE BEACH

White stones are often the first you see on a beach – they stand out among the darker coloured stones, especially when wet but also when dry.

Upon closer inspection, there seem to be a variety of shades of white, from almost clear (translucent) to dense and opaque. Some stones are completely white, other stones contain white spots or crystals, others seem to have white veins or patches. This Post looks 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. The next Post looks at stones with opaque white spots and crystals, and the Post after that considers stones with opaque white veins and 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.

1 – WHITE MILKY QUARTZ STONES

Below are three white quartz stones, W1, W2 and W3 (“W” stands for White) found on Gemstone Beach. They are opaque (light doesn’t shine through them) and are nearly purely white. These are sometimes referred to as cloudy or milky quartz, and there seems to be some very minor variations within each in terms of their opaqueness. As with many such beach quartz stones, these are spherical or egg-shaped, and quite smooth.

As I noted when looking at translucent quartz stones, 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 (these terms are explained in Part Four of this Series). 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 – and there are many more variations. Quartz is usually viewed as an igneous rock that crystallizes from magma or precipitates around hydrothermal vents. As Wikipedia notes: “Milky quartz is the most common variety of crystalline quartz. The white color is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation.” 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”.

Some white milky quartz stones contain small specks (Stones W4 and W5 below) or stains or patches of another mineral, often iron oxide. For example, the rusty iron oxide stains on the top of Stone W6 (below) are relatively common.

Sometimes white milky quartz stones can vary noticeably in material, such as those with patches of variable opaqueness and translucency, as with Stones W7 to W9 below (Note: Stone W9 is the same stone as Stone T41 in Translucent Stones):

Other variations of milky quartz exist – every white stone is practically unique.

2 – WHITE HYDROGROSSULAR GARNETS

Some white stones on Gemstone Beach kind of look like white milky quartz but have a different feel and a slightly different appearance. They are hydrogrossular garnet stones, with Stones W10, W11 and W12 (below) being good examples:

Note that Stone W12 (above) has some thin translucent-looking veins as well as a couple of bright white opaque veins. Stones W10 and W11 lack such veins. Often small white pieces of such stones, the size of a fingernail, sit shining brightly on the beach. Gemstone Beach is well known for hydrogrossular garnets and is probably the beach on which they can be most easily found in New Zealand. Despite that, they don’t always show up. 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 garnets often feel waxy, and they have a slightly duller look than white milky quartz. Some exceptions exist and will be noted below and in other Posts. When wet, water tends to “bead off” the waxy surface of these stones, forming beads or droplets that roll off. The stones that shine most bright on the beach are white milky quartz and white hydrogrossular garnets.

Hydrogrossular stones come from a metamorphic rock called rodingite which has other minerals mixed in as well. Sometimes a tiny bit of other material is incorporated into the white opaque hydrogrossular garnet stones. Black or gray is common (as with Stones W13 and W14) as is yellow (Stones W16 and W17 are good examples). Also note that W16 is an example of a denser white form of hydrogrossular garnet, while the lower part of Stone W17 appears to be almost colourless.

Some white hydrogrossular garnets have quite a variable character, with patches of different shades of white, some tiny areas or veins also looking translucent. Stones W18 to W21 are examples:

There is another kind of white hydrogrossular garnet on Gemstone Beach that is very opaque and which often does not have a waxy feel. It is one of the most common variations to be found there. In my TumbleStone Two Post on the seven main types of hydrogrossular garnets, they are listed as the fifth type, “white stones with dark intrusions”. In Thornton’s entry on “Garnet” on page 9 of her book on “Gemstones”, at the top left of the photo of stones and numbered “1” is a photo of such a stone. Thornton refers to it as “a stone from the beach at Orepuki, showing a dark crystal of diopside in the garnet.” The diopside has a black or dark gray or silver appearance. In Stone W14 (above), it might be diopside that is lurking just under the surface at the bottom of the stone, and Stone W21 (immediately above) shows such a dark silvery crystal. Below are three examples of the dense white hydrogrossular garnets with diopside, Stone W24 having a number of smaller black inclusions:

Stones W25 and W26 are maybe also versions of this, with much smaller diopside crystals in them – again, these stones often don’t have a waxy feel, but I have learned they have hydrogrossular garnet in them by talking to other rockhounds in person and on the internet:

Sometimes the white opaque variety of hydrogrossular garnets has a pink blush in them, most likely due to thulite being present. Examples will be provided in the Post on “Pink Stones” (not yet available).

3 – WHITE IN OTHER-COLOURED HYDROGROSSULAR GARNETS

Hydrogrossular garnets come in a range of colours. For example, in her book “The New Zealand Rockhound” (1981), Natalie Fernandez notes on page 50: “Hydrogrossular garnet is a massive form of calcium garnet…Colour: A wide range, patchy. Pale green, greyish, yellow, white, brown. Occasionally shades of orange, darker green, even blue and pink.” Some of the coloured hydrogrossulars also contain some white – see three examples below. These three stones have a waxy feel that help to identify them as hydrogrossular garnets.

Note that, in general, hydrogrossular garnets are not common on Gemstone Beach but can occasionally be found there. We will come across hydrogrossular garnets again in Posts on other colours.

The next Post in this Series looks at two further groupings of white stones that can be found on Gemstone Beach, stones with opaque white spots and stones with opaque white crystals.

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

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