Doug from Whanganui’s River City Press wanted to interview me about stone collecting for a local newspaper (see “John Paterson’s hunt for coloured stones”). So we chatted for an hour or so in a local cafe one morning. I brought along some of my polished stones to show him and later he asked to buy some.
I have picked out 30 Gemstone Beach stones for Doug. This Post features the first five, the largest, along with links to Posts which say something about them. Part Two describes Stones #6 to #11; Part Three is about Stones #12 to #17; Part Four features Stones #18 to #23; and Part Five looks at Stones #24 to #30, the seven smallest. But first, Gemstone Beach is introduced.
Gemstone Beach is located just outside of the small Southland town of Orepuki, 70 kilometres west of the city of Invercargill. It lies towards the eastern end of Te Waewae Bay, a long sweeping bay of some 27 kilometres, part of Foveaux Strait between the South Island and Rakiura Stewart Island. There is a great diversity of colourful stones that can be seen on Gemstone Beach, many of them brought down to the coast from the Eastern Fiordland mountains by the Waiau River. A lot of the stones could be called “semi-precious gemstones”. “Precious gemstones” are ones like emeralds, rubies, sapphires and diamonds – you won’t find these at Gemstone Beach. “Semi-precious gemstones” are stones that look beautiful or interesting, that could be polished and made into a pendant, for example, but are not worth very much. I drive down to Southland from Whanganui two or three times each year to find stones to take home to tumble polish. This Post is a retrospect on my most recent trip.
The 30 polished Gemstone Beach stones I picked out for Doug (see photo above, far left) represent a glimpse into the diversity of stones to be found on the beach. In the photo, they are arranged from largest to smallest and are numbered. Stone #1 is 4.5 cms by 4 cms, Stone #30 is 2.5 cms by 2 cms. This Post features the first five of the 30, with the rest covered in later Posts.
Stones #1 and #2 (above) are black igneous stones with white crystals. Stone #1 is a “porphyry”, meaning the white crystals in it are relatively large, compared for instance with Stone #2. The white crystals are probably feldspar and the black is perhaps basalt. A number of similar black and white stones can be found on Gemstone Beach. Their crystals have formed as molten magma has cooled – atoms and molecules group together and “grow” into larger groupings, just as ice forms in freezing conditions. See The Conversation for an account of crystalisation in igneous rocks. The size of the crystals, their shape, and the ratio of white to black can all vary quite a bit. It depends on how quickly the molten rock has cooled and hardened, and what minerals are present to make up the crystals. There are two types of igneous rock, volcanic and plutonic. 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. A porphyry, like Stone #1, has well-formed crystals visible to the naked eye (called “phenocrysts”), set in a very fine grained matrix. For more, see the second half, called “5 – Stones with Opaque White Crystals”, of “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Six-B, Stones with White Spots & Crystals” (15 September 2023). Stone #2 is also a black igneous stone with white crystals in it but the crystals are a lot smaller. This means the stone cooled more quickly as it hardened, shortening the time available for crystals to grow. Stones like #1 and #2 tumble polish well because they are hard and the crystals are well embedded in them.
Stone #3 (above) is a green sedimentary stone, most likely argillite. There are eight argillite stones in Doug’s 30, the others being Stones #6, #7, #10, #12, #14, #19 and #20. In Stone #3, layers of green sediment of different intensities have been compressed together to create a form of banding in it. The “bands” could almost be called “veins”, made up of an intense green mineral. Argillite is a mudstone, consisting of much smaller grains than, for instance, greywacke sandstone. However, it has been hardened more than other sedimentary rocks, having been subject to a bit more heat and pressure. This means it tumble polishes better than most other sedimentary stones. Green argillite is very common on Gemstone Beach, and a red variety can also be found there. “Pakohe” is the Māori name for a grey argillite traditionally used to make tools, weapons, and other objects. It was valued for its hardness, strength, and ability to hold a sharp edge (The Prow). See “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Seven-A, Green Argillite Stones” for more – Stones Gn8 through to Gn13 are similar to Stone #3.
Stone #4 (above) is what I call a sugar quartz stone, and it is coloured by iron oxide. Quartz is a form of silica and is one of the most common minerals on Earth. 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. The New Zealand geologist Jocelyn Thornton, in her booklet “Gemstones” (1985), notes on page 10: “Most quartz found will be massive, composed of small, interlocking crystals. Pebbles of this material are the most widespread pale stones on our beaches.” 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 examples of milky quartz from Gemstone Beach, see Stones W1 to W3 in Section 1 of this Post. Stone #4 is not a milky quartz but is a more granular or sugar-like form of quartz. The iron oxide that colours it brown is a very common chemical compound in a wide diversity of rocks. This “rusty” compound of iron and oxygen often precipitates from heated iron-rich groundwater as it cools. Its presence can lead to yellows, oranges, reds, browns and blacks.
Stone #5 is a very dense and opaque form of milky quartz. It is uniformly white. It lacks the granular character of Stone #4, as well as its iron oxide staining. When found on the beach, few of these stones are totally smooth, often having a small chunk out of them or a deep scratch. Through tumble polishing, a smooth specimen, like Stone #5, can often be achieved.
The next Post in this Series features Stones #6 to #11 of Doug’s 30.
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