I found these stones on Gemstone Beach in February/March 2021. Each stone is really interesting, but two have polished very well and two have minor issues with them.
I found the first stone on the afternoon of Sunday 28 February 2021. I arrived at high tide that day and the waves were initially lapping the cliffs high up on the beach, forcing me to spend the first 40 minutes near the Taunoa Stream, where I fossicked in the stream’s shallow water. This stone, found there in the stream, was one of a handful of outstanding finds that day. It has amazing lines and patterns in it, but I don’t know what kind of stone it is.
The stone is a reasonably big one as far as tumble-polishing goes, being 6 cms by 5 cms. Along with the other three stones, it was tumbled in a 6lb barrel which is able to deal with this size. Initially it spent just over a week in 400 silicon carbide grit to smooth it in preparation for polishing. It was then tumbled in pre-polish and pro-polish tin oxide followed by a two days’ burnishing tumble in borax. The first four photos below are of the stone as found on the beach, with four comparative photos of the polished stone beneath:
The stone polished very well. It is very smooth and has no rough areas. In some ways, however, it is not really a significant improvement on the wet stone as found on the beach – except for one major thing. A polished stone is always bright and clear. A stone as found on the beach shows its colours only when wet – when it is not wet, it is dull and does not reveal its intricacies and intensity.
The second stone was found on Gemstone Beach on Thursday 4 March 2021. It is a larger stone, being 7.5 cms long, again requiring a 6lb barrel to polish. It was the myriad shapes of green hues that caught my eye when I came across it on the beach. The stone is a sedimentary one, probably a mudstone, and the shapes within it are probably some kind of infilled holes or burrows. It is not always possible to achieve a smooth surface on such stones when tumbling them as the infills may not erode or polish at the same rate or to the same degree as the rest of the stone. But this one has turned out great! It is smooth all over.
When you compare the before and after stones (top and bottom respectively), there is not a lot of difference, except that the effects of tumbling can be seen in the often subtle wearing away of the infill shapes.
The third stone, about 5 cms long, was found on Gemstone Beach on Saturday 6 March 2021, the day after a tsunami alert. It may be a volcanic stone which started out with small holes (vesicles) which have been infilled as water has moved through the holes and left minerals behind. These infilled holes provide the stone’s beauty but also pose a challenge for the tumble polisher. The holes may be filled with material that wears away at a different rate than the stone itself, and some may not even take a polish.
While the light-coloured original material of the stone has polished well, the infilled holes show a less favourable outcome. Many of them look clearer prior to tumbling. I found this disappointing. I have had mixed success with stones like these – sometimes a good polish can be achieved for the whole stone. It depends on the nature of the infill mineral. I do wonder whether being tumbled in a finer grit – say 600 – might improve the end product.
The fourth stone was found on Gemstone Beach on my first visit there during my February-March 2021 trip, on Sunday 21 February. The original photos of the stone were taken not on the beach when I found it but back at the crib/bach I was staying at in Riverton. About 6 cms long, this stone is much darker in colour than the third stone but it also has a number of interesting infills. It also has a hole at one end, on one side, with a crack running outwards from it.
The main dark material of the stone has polished well along with the vast majority of the infills. Some changes can be seen in the shape of the infills, due to the wearing arising from tumbling in fine grit. The hole remains, basically unchanged. To remove the hole by tumbling in a coarse grit would reduce the size of the stone significantly and I decided not to do that. Such a hole actually reveals the internal nature of the stone and can sometimes be quite interesting. The crack has maybe become a little more pronounced. Overall, I am pleased with the polish on most of the stone.
Tumble polishing is an art, not a science. It is imprecise, dependent on the stone and its constituents, the starting conditions of the stone, and the skill of the tumble polisher to follow various steps carefully. It also depends on what the tumble polisher wants to achieve with a particular stone. Tumble polishing usually leads to amazing clarification of the surface of a stone, a clarification that does not simply fade away in a few minutes as happens with an unpolished stone that is wet.
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