The Tauranga Gem and Mineral Club is a well-established and active club. Its website mentions that it is located at Tauranga’s Historic Village where it is home to a mineral museum featuring a comprehensive display of rocks and minerals. Also there is a flourescent room displaying rocks that glow in the dark, and a shop that sells work crafted by the members from rocks and semi-precious stones. The Club has a weekly workshop night on Tuesdays and a regular meeting night on every second Monday of each month. It also has an active field trip programme. Its latest field trip brought a number of club members to Kai Iwi Beach, visiting there and nearby Ototoka Beach mainly for fossils. I live close to the Holiday Park they stayed at and went along to share some of my collection of Gemstone Beach stones.
One of the field trip organisers had contacted me to let me know the Club was going to be at the Holiday Park this weekend. I volunteered to bring some Gemstone Beach stones along if members might be interested in them. I thought that people might like to see and pick up a range of stones from a well-known beach at the other end of the country, on the south coast of Southland. There’s nothing like holding a stone in your hand. And I had a couple of trays with water in them so people could wet a rough stone to see the colours in it better.
About 17 or 18 people filled up the Holiday Park’s camp lounge on Sunday morning. I briefly introduced myself and talked a little about TumbleStone Blog and how I take photos of stones. I said a little about Gemstone Beach and fossicking along the Te Waewae Bay coast. For information on Gemstone Beach, see “Gemstone Beach and Its Stones: An Introduction for the Passing Motorist – Part Two, Main Features of the Beach” and “Gemstone Beach: Location on Te Waewae Bay”. It turned out that maybe 80% of the people there had visited Gemstone Beach at some point. I brought along three plastic boxes of stones – each box had eight compartments. I had 12 groups of stone types, with separately housed rough and polished specimens.
The four groupings of stones in Box One were poppy jaspers, pinks (thulite and rhodonite), amygdaloidal stones, and reds (including hematite jaspers):
Poppy jaspers are a kind of orbicular jasper, having red or orange orbs that look like poppy flowers. Orbicular jaspers form due to the presence of minerals which crystallise in concentric layers around a nucleus or central point, giving rise to orb-like structures. Sometimes the orbs are red or orange, but they may be other colours. The poppy jaspers I find on rare occasions on Gemstone Beach tend to be quite small and often have pits and holes in them. They are fascinating when seen in close-up photos. See “O is for Opaque Orepuki Orbicular Jasper” and “A Small White Orbicular Jasper From Gemstone Beach”. The poppy jaspers attracted a lot of attention from people at the gathering this morning. The second set of stones in Box One consisted of a couple of types of pink stone. The main pink stone found on Gemstone Beach is thulite, though again it is not common – see “January 2022, Stone of the Day #5 – A Little Pinky”. Thulite is the pink to reddish variety of the mineral zoisite. Its pink hue is caused by the presence of manganese. Pink rhodonite is slightly less common on Gemstone Beach – see “A Rhodonite Find on Gemstone Beach”. Box One, Stone Three, amygdaloidal stones, arise when molten magma solidifies with tiny holes in it, little gas pockets. These holes then fill with mineral-rich fluids which leave behind deposits of minerals such as quartz, chalcedony, and calcite. The resultant “spots” are called “amygdales” – see the first part of “Gemstone Beach and Its Stones: An Introduction for the Passing Motorist – Part Six-B, Stones With White Spots & Crystals”. The last group in Box One were a number of red stones, many of them jaspers including hematite jaspers – see “H is for Hematite Jasper” in this Post.
Box Two had hydrogrossular garnets, trace fossils, ignimbrite (rhyolite?), and chromium in quartz:
Gemstone Beach is especially known for its hydrogrossular garnets, a non-crystal type of garnet. They shine brightly when wet and typically have a waxy feel. There is quite a range of colours and kinds – see “Hydrogrossular garnet”. The second group in Box Two was trace fossil stones, and they attracted the most interest in the questions and discussion this morning. Trace fossils are not fossils of an animal but are fossils of the traces they leave behind. Those traces result from such activities as burrowing, moving, excreting, fighting and feeding. The Gemstone Beach trace fossils occur in argillite, a hardened mudstone, which was laid down in the Permian Era, 250 to 300 million years ago. There are a wide range of trace fossil stones that are reasonably common on Gemstone Beach. See the last part of “Gemstone Beach and Its Stones: An Introduction for the Passing Motorist – Part Seven-A, Green Argillite Stones”. Ignimbrite is the third group in Box Two. NOTE, 10 September: This identification of these stones has come into question – they might instead be metamorposed banded rhyolite. The following is what I wrote in this Post originally when believing them to be ignimbrite. These stones originate in volcanic ash fall. When the fall lands and accumulates thickly on the ground, eventually it compresses and turns to rock. Ignimbrite is fine-grained and glassy and can tumble polish well if there are few small pits in it. See “Stay-at-Home Day Six, Tuesday 31 March 2020: Stone Six”. Box Two, Stone Four is chromium in quartz, by far the smallest group of the 12. This is the stone whose identity I am least sure of at this point. I used to think it was hydrogrossular garnet but two or three sources have suggested chromium in quartz. It is a very glassy stone and I get the impression it would shatter if struck with a sharp blow.
Box Three had greens, black and white igneous stones, argillite (including spotted argillite), and banded argillite:
The first group in Box Three is greens. There are many green stones on Gemstone Beach (though no greenstone). Most of them are coloured by epidote and include argillite and quartzite. Group Two is black and white igneous stones. I find these very interesting because of the range of types and sizes of white crystals in them. And they usually tumble polish very successfully. There’s a section on “Stones with opaque white crystals” in “Gemstone Beach and Its Stones: An Introduction for the Passing Motorist – Part Six-B, Stones With White Spots & Crystals”. Box Three, Stone Three is argillite, including spotted argillite, and Stone Four is banded argillite. On Gemstone Beach, argillite stones come in green (by far the most common colour), gray and red, with some banded argillites being brown as well. The spotted variation seems to arise when argillite comes under increased pressure and heat, more than is usual for a sedimentary rock. In other words, it might be one of the first signs of argillite becoming metamorphosed as certain minerals become concentrated in small areas. Banded argillites are made up of a number of thin bands of different colours. Some brown and grey banded argillites also appear to have been subject to more heat than usual during formation, their bands looking as if they melt into each other, like pottery glaze. They shine brightly on the beach when wet, and can take a very good shine after polishing. More information on the different types of argillite can be found in “Gemstone Beach and Its Stones: An Introduction for the Passing Motorist – Part Seven-A, Green Argillite Stones”.
I also took along three small containers of other stones (see photos below) – I put these on the table but people may not have seen them. Two containers held some stones that I had collected from Gemstone Beach that I had not yet identified with any certainty. Some were rough and some were polished. I also had a few polished stones from our local beach, Kai Iwi Beach – see “The Iron-Stained Stones of Kai-Iwi Beach” for information on the kinds of stones to be found here.
I enjoyed the challenge of organising groups of different stones and finding rough and polished specimens of them. Most of the rough stones were from my August fossicking trip. The polished stones had been collected over the past four to five years.
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