See here for the first Post in this Series.
Day 19, Sunday 7 March – Travel day. I drove north from Riverton to Kakanui (near Oamaru), and made a brief reconnaissance visit to Kakanui Beach (planning to spend two days here). I arrived at my favourite Kakanui Beach late afternoon and spent about 30 minutes checking out what stones were around. Here are five that I found:
Day 20, Monday 8 March – Kakanui, Moeraki Village and Katiki. To start off with today, I spent 3 1/2 hours on the beach near Kakanui.
I found quite a few of the yellow and red quartzites that I like. I first came across a type of yellow quartzite at Birdlings Flat, near Christchurch, a few years ago and was pleased to discover even more of them appearing in the Kakanui area. Some have some red in them, and some I thought have almost no yellow (or red) at all. Here are 11 of these stones found today, starting with predominantly red ones, moving through to yellow ones then to increasingly plainer ones. Yet all the same type of quartzite, I reckon.
Among the other Kakanui stones I found today were these seven:
And these seven:
In the afternoon I tracked back south for about 30 kilometres to visit a small beach not far from Moeraki village, a recommendation from Oliver who had earlier introduced me to Slope Point.
Oliver said I might find some agate there. And I did – botryoidal agate, seam agate, and sea-tumbled beach agate.
As noted by Jocelyn Thornton on page 11 of “Gemstones”, this form of cryptocrystalline silica is more accurately called “chalcedony”, with “agate” being used for the banded variety (see also Te Ara: The Encyclopedia of New Zealand). The Quartz Page provides a well-considered view on this. However, in New Zealand most varieties of chalcedony are often simply called agate. (Malcolm Luxton in “Agates of New Zealand” refers to some types of non-banded chalcedony as agate, such as those with mineral inclusions.)
Chalcedony is composed of very fine intergrowths of quartz and moganite. It is microcrystalline meaning the crystals are microscopic and cannot be observed by the naked eye. Both quartz and moganite have the same chemical formula SiO2 (silicon dioxide) but different crystal structures. When free from impurities, chalcedony is colourless and transparent. Dependent on impurities introduced during its formation, chalcedony comes in a wide variety of colours including red, yellow, green, blue, purple, grey, and white.
I was astounded to find the piece of botryoidal chalcedony/agate, having never come across it before. It was partly buried in the sand and I missed it first time I walked past. I went back a few paces for some reason to look at something else and noticed it from that direction.
As one website puts it, “Botryoidal minerals form when many nearby nuclei, specks of sand, dust, or other particles, are present. Acicular or fibrous crystals grow radially around the nuclei at the same rate, appearing as spheres. Eventually, these spheres abut or overlap with those that are nearby.” The Moeraki/Kakanui area is well known to have agate/chalcedony, as indicated on the map of rock collecting sites in Te Ara (see photo on left below). The Otago Rock and Mineral Club have a photo of a botryoidal chalcedony with a movable cap from the Moeraki area and a piece from Moeraki sold last year from the Trevor Gray collection (photos centre and right below).
Seam chalcedony or agate is a form of this type of rock where layers build up to fill cracks and cavities in sediments (instead of occurring in holes left by gas bubbles in volcanic rocks). Jocelyn Thornton’s “Gemstones” includes an example on page 11. Craig McGregor has an interest in a particular type of seam agate found on the beaches on the south side of the Moeraki Peninsula. He calls it “Tobacco Agate” because of its very streaky layering. Again, I was astounded to find a good-sized piece of seam agate high up the beach, with very few other stones around it.
The sea-tumbled beach chalcedony/agate I found might have started off as a seam but through wave action, being rolled against sand and other stones, it became shaped into a rounder form. You can often see tiny “concussion” marks on the surface of beach chalcedony, caused by being slammed into rocks and stones by waves and/or the current. Such marks are most noticeable when the stone is dry.
After my visit to this small beach near Moeraki Village, I went south another few kilometres to the Katiki Beach North Reserve Rest Stop. I had noticed some stones on the beach there yesterday when driving past. A scattering of stones was to be seen across pa lot of the beach. A 20 minutes fossick turned up a handful of interesting stones, and the potential existed for more, but the afternoon was wearing on and I headed back north to Kakanui and my accommodation.
The next Post in this Series, on Days 21 and 22, is here.
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