Just before Christmas, I finished polishing some of the stones I had collected from Gemstone Beach and nearby in early 2023. They were tumbled in two 3lb barrels, the final stage for each batch being a tumble in tin oxide polish for just over two weeks, followed by burnishing in borax (for three days for one barrel, for five days for the other). Most of the stones would have been tumbled prior to this in 600 grade silicon carbide grit; all of them were tumbled in 400 grit before that; and some of them, those needing the most smoothing, would have been first tumbled in 220 grit.
In this Series of Posts, I present 30 of the smaller stones, to show their character as well as the difficulty of tumble-polishing. Throughout the stages of tumbling, I carefully sort through stones to try to make sure that they are as smooth as possible before they go on to the polish tumble. At first glance, I thought all the stones in these two batches were nice and smooth, and that they ended up well polished. However, looking at these photos, I can see the need for me to be even stricter before the final stage, as tiny holes, bruises, scratches and rough patches are apparent on many of the stones. I need to pay more attention to cushioning filler – I am currently reducing my use of plastic beads and using very small stones instead. I didn’t seem to get the balance of filler to stones quite right with these batches. Nevertheless, what is also apparent is the often complex and beautiful character of these stones, truly natural wonders from a Southland beach.
The first five stones are the larger of the 30, though the largest, Stone 1, is only about 5 cms wide. It is an example of a stone made up of lots of different little bits. In terms of appearance, it has many shades of grey. In constitution, it is a breccia, meaning that it consists of angular fragments of stone that have been compressed together in a matrix – see University of Auckland Geology. [If the fragments, known as clasts, were rounded, it would be a conglomerate, the fragments having been eroded by travel from their source before being compressed.]
Some of the fragments are likely to be argillite, a mudstone, but there are a lot of other little things in there too, various grains and crystals. Some of the crystals reflect the light when held at an angle. This diversity of material in the stone also explains why it did not take a uniform polish – for example, some of the fragments, grains or crystals will be softer.
Stone 1 is a dark stone, but Stone 2 is much lighter in appearance (though with a few dark patches).
One of the black patches on Side B is particularly striking. Again, this stone is a breccia, with the dark patches being angular clasts. Much of the light-coloured material could be quartz of some form. Quartz is a form of silica and is among the most common of all rock forming minerals. It is found in a wide variety of types in different kinds of rocks. It tends to tumble polish well.
Stone 3 is spotted argillite, a fine-grained mudstone. The “spots” are not perfect, many are not well-formed, but the colour variation they introduce leads to an interesting visual pattern. On Gemstone Beach, argillite is often a solid green colour, but this is an example of one of the variations that occurs. The green of Stone 3 is highly likely to be caused by the mineral epidote which in New Zealand is widespread in veins in greywacke, schist and granite. Epidote is in fact the main mineral in the iron-rich mudstones of Otago and Southland, including argillite.
Stone 3 has come through the tumbling process reasonably well, though there is a slight scratch on Side B. In this Post on green stones on Gemstone Beach, 12 other specimens of spotted argillite are presented (Stones Gn20 to Gn31). I suggest there that “spots” occur when argillite comes under increased pressure and heat, more than is usual for a sedimentary rock. During this process, certain minerals become concentrated in specific areas.
Stone 4 is probably of igneous origin, most likely volcanic. It is quite different from the previous three.
It is mainly dark, yes, and has a mixture of material in it, including white crystals (or are they clasts?), but it also has some intriguing subtle linear shapes. It was those shapes that caught my eye on the beach, and it is their existence that defines the individuality of the stone.
I found Stone 5 on 12 April 2023 (see this Post). It appears to be made up primarily of black and white crystals, with minerals providing pastel colour patches of orange/pink and light green.
The black crystals help create contrasting halves in the stone, with the pastel colours providing a subtle delicacy. The orange/pink is likely to be due to the presence of orthoclase feldspar, a potassium-bearing member of the alkali feldspar family – it is orthoclase feldspar that gives granite its pink component. The green patches are likely to be caused by the mineral epidote (mentioned in relation to Stone 3 above). Stone 5 has tumbled well, though Side A has some tiny rough areas.
The next Post in this Series examines Stones 6 to 10.
Once more, I’ve learned another thing from you – breccia / conglomerate and why it’s called one and not the other! Thank you!
Was also wondering if you were thinking that the orange/pink of stone 5 was feldspar and the green, epidote?
Good point about the colours. Will add that to the Post. Thanks!