This is the 13th Post in this series on the tumble-polishing of 40 Riverton stones, which starts with stone collection and goes right through six different tumbling stages. The first Post can be found here.
The previous Post examined how Stones six to ten came through the tumble-polishing process. This Post looks at Stones 11 to 15, discussing how many of them experienced some damage but still polished well enough to come out looking very interesting:
STONE 11
Stone 11 is probably a sandstone. In general, it has polished very well, and details of the sand grains that make it up have become clearer to see. This, along with the thin red band and darker base, makes it an interesting stone.
There are some problems with it, however. When I inspected it after tumbling in 100 grit, I noted that it had a very minor superficial crack on one side, near its base. After the 220 grit tumble, this had been reduced. This crack remains, though in a reduced state, and can barely be felt when I rub my thumb over it. There are three other similar superficial cracks on the base of the stone, in the dark area. I had seen them at the inspection of the stone after Stage Two but did not think them to be significant. They were not noticeable after the 320 grit tumble. However, they have reappeared, including a long one along the base, and a smaller one on the side can be felt more distinctly than the others.
There are no “imperfections” in the rest of the stone, which is lighter in colour – maybe the darker section is just a little more brittle. The fact that these superficial cracks are still there after the various tumbling stages suggest that further tumbling in, say, 320 grit may not remove them. Maybe it would take a 220 tumble to achieve this, at the expense of making the stone a little smaller.
STONE 12
Throughout the tumbling process, Stone 12 exhibited some fragility. Small chips were lost from it now and again, caused by bumping into other stones in the barrel. Even in the pre-polish barrel, it lost two or three small pieces. This is a pity because it is a gorgeous stone with a very interesting make-up. It has an obvious crystalline character which means it is probably quartz, stained with shades of cream. And apart from where it was chipped, the stone has polished very well.
I put Stone 12 through to the final polishing stages though normally I would have discarded it or returned it to a 320 tumble once or twice. As a result, there are distinct imperfections with the final polished stone.
STONE 13
Stone 13 has polished very well. There are no scratches or holes obvious to the eye to when I rub my thumb over it. Polishing has brought out a complex pattern of colour, grayish purple patches drifting on a green background, with white quartz veins slashed across it.
When a dry stone, straight off the beach, Stone 13 had a weathered dull look, though when wet it revealed some of its promise (as can be seen in the photos of Stone 13 in The Seven Stages in Tumble Polishing Stones: The Selected 40 Stones). My guess is that Stone 13 is some kind of metamorphic stone but I struggle to identify it further than that.
STONE 14
Stone 14 is pink granite, though at the darker shade end. This identification is made partly on the basis of information provided by Te Hikoi The Riverton Museum. It has developed an outstanding chidren’s rockhounding kit which includes pink granite as one of the kinds of stones that can easily be found along the south coast of Southland. The museum has a display supporting the kit, and has a set of drawers with rock samples in them.
One of the problems with granite, for the tumble polisher, is that it consists of grains of feldspar, quartz and mica. Depending on how tightly these grains are packed together, the tumbling process may dislodge some as the granite stone bumps against other stones. Furthermore, some of the grains may not polish, or may not polish as well as other grains, preventing the achievement of a consistently polished surface. In the case of Stone 14, the surface feels smooth to my thumb but a close visual inspection shows tiny parts are unpolished. Small chips have also been dislodged from both ends of the stone.
Tumble polishing has made the character of Stone 14 shine out clearly but it is not a perfect polish. Granite often poses this challenge for tumble polishing.
STONE 15
I observed from the outset that Stone 15 had a small gouge in the side. This gouge got progressively shallower and more smooth as the stone with through the three grit stages but remained larger than I would normally have liked when it went to the polish tumbles. However, the gouge was an indicator of the brittleness of the stone – a close inspection reveals a number of other very small cracks and chips in the stone. They were not as apparent earlier on. It is hard to know whether further tumbling with 320 grit would remove them.
The polish on Stone 15, which is probably some kind of mudstone, is not of the highest quality but again has revealed its patterns in detail.
Wow, I just found this blog series today, as I was trying to figure out if my first batch of stones need to go through the 100 mesh grit stage (they’re quite rounded beach rocks) – I’m so excited and pleased that this series is still on-going. As I begin to tumble rocks of my own (my new tumbler is coming in the mail this week), I look forward to following along with the rest of the series, and to see the outcome of the rest of your stones!
Great! That’s exciting! You will find it really interesting to see how stones turn out after tumbling. As I mention in my blog, I often start smooth beach stones at the 320 grit stage. If they have a chip out of them or a scratch or some roughness, then maybe 220 grit is better first. Ultimately, you have to learn by trial and error for yourself. I started off by following instructions religiously before trying my own variations and short-cuts once I saw how things worked. I will be interested to hear how things go for you. Best wishes, John.
Thank you so much for the detailed, step-by-step instructions! Makes my OCD heart so happy! I have been looking for something just like this.
Thanks, Debbie, for your encouragement. I am pleased you have found this useful. Just remember that it is one person’s views and experiences, so if you think after a while there might be a better way to do something, go ahead and try it.
Thank you so much taking the time to document and post your process. I am new to rock tumbling and my first batch of rocks is from glacial till on the North Shore of Lake Superior in Canada. These rocks were already smooth and rounded so I started them in 80 grit, then 220.
I am now at the pre-polish stage and could not find any good information on how they should look coming out of this stage until I found your blog. None of the videos, etc. had any specific information except to say a “smooth” or “matte” finish. As I am using a vibratory tumbler for this stage, and various sources give widely different time estimates, I needed to know what the rocks should look like. Thank you for your excellents pictures and descriptions!
Hi! Glad my Blog helped you. It was good that you were able to adapt the information for your context.