Three more of my recent regular Posts in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. The first Post in this Series is here.
Wednesday 5 May 2021: Banded argillite stone from Gemstone Beach, size around 3.5 cms. Tumble-polished.
Argillite is a hardened, slightly recrystalised, mudstone, mainly grey through to black in colour but can also be green and red (arising from the presence of iron). Green is the most common colour for argillite on Gemstone Beach (with some having trace fossil burrows and trails in them) but some very nice grey banded stones can also be found. This particular stone is unusual as it has a subtle brown hue. Its “bands” or layers show signs of pressure deformation, with the beginnings of a stress fracture line apparent on one side. Tumble-polishing has smoothed the stone well and clarified its colours, though it has not taken a high polish.
Thursday 6 May 2021: In his book “Terrain: Travels Through A Deep Landscape” (2015), Geoff Chapple recalls a walk with the geologist Nick Mortimer along the Southland coast between Colac Bay/Oraka and Riverton/Aparima. On page 253 he writes: “We looked for the tattooed rock, the trace fossils that Maori call mokomoko, and it took some time but we did find them, wetting down the face of a rock layer to reveal 270 million-year-old traces of burrowing worms that took the purple of their starting layer down into the pale depths beneath, or dragged their pale layer into some purple darkness below, working their primitive palette, thousands of small finger painters out of the Permian…” These two tumble-polished trace fossil stones illustrate the dark in the light and the light in the dark (though there is little purple there).
The stones are mudstones, probably argillite, and the burrows and trails left in the ancient ocean floor sediment by these animals millions of years ago have been infilled by much finer sediment (silt?) of a different hue. The sediment has been compacted and cemented, producing this sedimentary rock and preserving the traces – further heat and pressure forms a different rock type (metamorphic) and destroys the traces. I have found such stones on Gemstone Beach and at Riverton/Aparima and, more recently, my wife found two at Whanganui, on South Beach, while riding her horse there. I treat such trace fossil stones lightly in the tumbling process (only one fine grit tumble before polishing) to preserve the traces, and they usually don’t polish highly. But tumbling removes the weathered outer layer and keeps the colours and patterns clear.
Friday 7 May 2021: These three tumble-polished stones come from Birdlings Flat having been collected there at various times over the past four to five years. They are examples of a type of quartzite to be found there, characterised by layers of shades of grey and white.
A quartzite starts off as a quartz-rich sandstone, a sedimentary rock that is grainy and feels like sandpaper. When the sandstone is exposed to high temperatures and pressures, the hard glassy metamorphic rock of quartzite is formed. Technically, more than 90% of the grains in a quartzite are quartz. Quartzite’s wide variety of colours (grey, red, green, yellow, and more) are a result of minor amounts of various minerals being incorporated with the quartz during the process of metamorphism. There are often tiny intricate veins of silica within quartzite stones, along with small clear crystal structures. Quartzites usually polish very well, due to their smoothness, hardness, and high levels of quartz, and their often complex patterns make them very attractive.
The next Post in this Series is here.
