The purpose of this and the following Posts is to examine how the 40 Riverton stones have emerged from the polishing process and to compare them with how they looked before polishing. The Seven Stages that these 40 stones have gone through are: Stage One, Stone Collection; Stage Two, Silicon Carbide 100 Grit Tumble; Stage Three, 220 Grit Tumble; Stage Four, 320 Grit Tumble; Stage Five, Tin Oxide Pre-Polish; Stage Six, Pro-Polish; and Stage Seven, Borax Burnishing.
This is the 12th Post in this series – the first Post can be found here.
To start with Stones 1 to 5 [Note: The second photo below does not do justice to the shine on the polished stones, due to the artificial lighting conditions under which the photo was taken]:
STONE 1
Before polishing, Stone 1 (far left) was about 8 cms long and about 5 cms wide at its widest point (using the graph paper under it) and after polishing it has lost perhaps only a few mms of size. I had noted in Post Two of this series that Stone 1 is most likely of volcanic origin, probably rhyolite, as it has small specks in it that would have originally be gaseous pockets. [Note: I have since decided that this stone is ignimbrite, not rhyolite.] After the 100 grit tumble, these small pits were obvious. I noted after both the 220 grit tumble and 320 grit tumble that further wearing away of the stone did not do away with these pits. They would prevent the stone from achieving a smooth polish over all its surface. Further tumbling in 320 grit would not have solved this problem. I have previously (and since) tumbled the same type of stone where the gaseous pockets appear to have been filled in with material and they have been very successfully polished over all their surface.
Stone 1 has polished well except for the many gaseous pits to be found across its surface. Close inspection with my magnifying glass also revealed some small cracks that had not been worn away and which I had not previously noticed:
This stone is in contrast to other rhyolites [ignimbrites] I have tumbled, where the gaseous pits have not been void:
Stone 1 is well-patterned and a nice large size. Even though it has failed to polish completely, it remains a fine specimen. The tumbling has brought out its pattern clearly.
STONE 2
Stone 2 is similar to Stone 1 in that it is of volcanic origin [probably rhyolite] and also started out with noticeable gaseous pits the size of which was actually a little larger than Stone 1. However, most of these pits were filled with material which resisted crumbling and which polished quite well. In general, Stone 2 polished much better than Stone 1. However, there are a small number of pits which did not polish and two small cracks that are apparent.
When rubbing a thumb across one side, a couple of these indentations are noticeable and interrupt the smoothness. Another tumble in 220 grit would probably have removed most of the pits and cracks, although there is no guarantee that all of them would have disappeared.
STONE 3
Stone 3 looks like a mudstone that has undergone metamorphic pressure. It is fine-grained and has lovely green patches and swirls. From the beginning it seemed free of pits and cracks and had polished very well. The beautiful patterns discernible in the rough stone have been revealed and clarified.
There is just a slight roughness along one edge, not very noticeable really, but otherwise it is excellent.
STONE 4
It was noted during the inspection of Stone 4 after the 100 grit tumble that it had a small indentation in it. I would normally have tumbled it again in 100 grit, although there is always the risk that this might reveal other soft spots. After the 320 grit tumble, I saw that the indentation had smoothed out a lot and thought that it might not affect the final polish. At the end of the polishing process, Stone 4 has turned out very well.
This stone is a breccia, composed of broken angular fragments of rock cemented together in a fine-grained matrix. Sometimes indentations in such stones are caused by a softer fragment being worn away more quickly than the rest of the stone. This is not the case here – the indentation is maybe a less compressed part of the matrix. It has polished to a significant extent but can still be felt when a thumb is rubbed over the surface of the stone. The only other notable thing is that the very edge of the point half way down the stone is slightly rough – another 320 grit tumble would have smoothed this out.
STONE 5
This stone has polished very well. It feels smooth all over. The eye can vaguely discern a few very small pits along one side which can be confirmed with a 3x magnifying glass. But these do not detract from the overall smoothness.
Stone 5 looks at first glance to be a metamorphic mudstone or sandstone but closer inspection shows it to include lots of small pieces of different things, including pits (filled in) that are normally found in a volcanic stone. Many of the stones I find cannot easily be identified by me.
The next Post examines Stones 6 to 10.
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