Since I got back home from Southland at the end of March, I have been tumble-polishing stones collected from Gemstone Beach near Orepuki (see here for an account of their collection). On 28 April the polishing process finished for one 4lb barrel containing a mix of different types of interesting and colourful stones. These are the subject of this Post.
These stones are not “gems” in the sense of precious and sparkling jewel-like stones such as diamonds and sapphires. But I actually find them to be much more interesting and intriguing, many of them a mystery as to how they got the way they are. In this sense they are the true gems of Gemstone Beach.
This batch of 95 Gemstone Beach stones started their tumbling in 320 grit and then were tumbled in pre-polish and pro-polish tin oxide before a final week in a burnishing tumble with borax. This is how they ended up:
To look at some of these stones more closely…
One of them is a gorgeous quartzite (my identification of it as a quartzite is based on what I learned at Vince Burke’s Museum at Birdlings Flat):
Quartzites are not as common along the south coast as at Birdlings Flat in Canterbury, but I always find a couple of excellent ones at Riverton and Orepuki. They polish well, often have great colour, have interesting veins through them, and there’s a depth to them that makes them intriguing.
Two of the stones look like marble. Again, I can usually find one or two such stones on a collecting expedition, and I have started to keep an eye out specifically for them:
Marble is limestone subjected to heat and pressure. Veined and patterned marble is often created when a pure white original marble is cracked or shattered and the spaces between the fragments are filled with other materials. As I noted, I don’t know whether these stones are marble or not, but they certainly remind me of it. Note: April 2025 – These stones are indeed NOT marble but are iron-stained quartz. They tumble polish well and often have interesting patterns.
Over the last three years, I have found maybe half a dozen stones like this one:
I have always suspected that they are petrified wood. However, I am beginning to wonder whether they are in fact a type of jasper. I have always found them in association with dark red jasper stones, at Waikaka and on the south coast, and sometimes paler forms of jasper have been nearby. They don’t have the obvious characteristics of petrified wood, like wood grain, but it has been their light brown colour that seemed wood-like. And they have a brittleness like jasper has. Irrespective of what they are, I like them. Note: April 2025 – These stones are indeed jasper, not petrified wood.
This next stone has a very unusual set of linear features:
It could be quartz with inclusions of some other mineral. Another stone with a different kind of “inclusion” is this small green one which has specks of pink:
The next four stones have various kinds of fascinating patterns. The first stone has a breccia section at the top. The second stone is about three-quarters red with various small inclusions. The third has alternating light and dark layers plus a white halo. The fourth is actually one of my three or four favourites in this group of 95, due to its patterns of shades of grey (the last four photos below are of this stone, and they do not do it justice).
One of the most unusual stones I picked up off Gemstone Beach in March is this one:
I think these are coral fossils in this stone. I have not done much reading on fossils but I have been recently been going through a book which I found in my local library, “Rocks and Fossils” by Arthur Busbey, Robert Coenraads, David Roots and Paul Willis (2007). On page 211, the authors mention that “corals are among the most common fossils on Earth”. They refer to “rugose corals” whose characteristics could account for some features of the fossils in this stone. Rugose corals have the following structure:
A fossilised coral like this would leave circular impressions of different sizes in a stone, depending how far each individual coral is eroded away from its top. Something like this would account for the patterns on the stone (above) I found on Gemstone Beach. Note: April 2025 – I suspect this stone is not fossil coral but rather an orbicular variety of some rock.
Sometimes I come across stones with pink in them. It is thought that the presence of manganese or cobalt could cause this. For example, rhodochrosite and rhodonite owe their pink colour to manganese, and cobalt ore is often pink. Here are two pink stones found on Gemstone Beach in March: Note: April 2025 – These are thulite.
Finally, five other interesting and colourful stones:
All of these stones illustrate the wide range of stones that can be found on Gemstone Beach that tumble polish well.
DEVELOPMENTS IN MY STONE COLLECTING AND POLISHING STRATEGY
I have become aware that I have changed my approach to collecting and polishing stones over the past three years. Initially, I collected any stone that looked like it might be interesting. I often thought to myself, “Let’s see what happens to this one.” I then tried to produce a completely smooth polished stone, with no flaws. I put each stone through all the stages of tumbling, starting with 100 grade silicon carbide grit. This meant that the stones lost a lot of their material by the end of the process. Along the way, I also discarded stones that had pits or cracks that were not worn away – I was looking for a perfect polish.
More recently, over the past year or so, I have come to collect much more selectively, and I polish with more leniency. When picking up stones on the beach, I now discard many of them even though I know they will polish well. They include, for instance, white quartz and red jasper, excellent examples of which I already have. I aim to collect the most unusual and interesting ones, and the ones that have very few pits or cracks, if any. I then sort through these stones when I return to my accommodation at the beach, dry them and inspect them carefully – flaws show up much more clearly when the stone is dry. I tend to discard maybe one-quarter to one-third of the ones I have collected so the ones I bring home to polish are much more likely to be interesting and to polish successfully. And I can start tumbling most of these stones with 320 grade grit, skipping two stages and saving at least two weeks of total tumbling time.
I am also less severe in judging an acceptably polished stone. I will now be happy with a stone that still has some flaws, that is not completely smooth. This means that I am able to keep a wider range of stone types. Also, the end product will be a bit bigger as it has not been worn away as much. A stone is kept based on its interesting colour or pattern rather than its smoothness.
