This is the 13th Post in this series on the tumble-polishing of 40 Riverton stones, going through each Stage from stone collection on the beach through to the final polish and burnishing. The first Post in the series can be found here.
The previous Post examined how Stones 1 to 5 came through the tumble-polishing process. This Post looks at Stones 6 to 10, giving consideration to how well they have polished:
STONE 6
Stone 6 is the type of stone that shows its character only upon tumbling. When rough and dry, it is unremarkable and inauspicious. Upon smoothing and polishing, its lovely patterns and grain become clearly apparent. There are some breaks in the surface of the polished stone, including chips at one end and on both edges. In all such instances, shiny mica crystals can be seen at the bottom of the chips. I had previously noted that one of these chips occurred during the Pre-Polish tumble, and the others would have been the result of the Pro-Polish and Borax tumbles. They would have been caused when other stones bumped into this one in the barrel, even though I used a lot of plastic beads at these Stages. Tumbling again in 100, 220 and 320 grit might erase these chips but might cause others. The stone has a slightly brittle character – maybe its metamorphism was too brief to bind all the constituents together tightly.
I have always felt that it is very difficult to guarantee a good smooth polish with this type of stone, though it is possible on very rare occasions and I have done it.
Stone 6 is a foliated metamorphic stone, somewhere along the schist-gneiss continuum. As a source on metamorphic rocks puts it: “Schist is a high degree of metamorphism, characterized by coarse grained foliation and/or lineation, with mica crystals large enough to be easily identified with the unaided eye. Gneiss is a medium to coarse-grained, irregularly banded rock with only poorly developed cleavage. The light and dark bands (gneissic banding) are alternations of felsic vs. mafic layers.” Based on grain size and type of banding, my guess is that Stone 6 is gneiss, while the stone below (which I recently collected on the Back Beach at Riverton and which has gone through a 320 grit tumble) is probably schist.
STONE 7
This is a gorgeous green mudstone, probably argillite, that has polished without blemish. From the wearing away of the surface, a broad light-coloured stripe has been revealed. This is in fact what is commonly referred to as a “fossilised worm cast”, left behind millions of years ago as a worm-like animal maybe crawled through mud, ingested material and then excreted it. As sediment layers have piled on top over many many years, and weight and heat built up, the process has turned mud and worm cast into rock.
Fossil worm cast stones can readily be found on Riverton beaches though they are more often associated with Gemstone Beach, near Orepuki, 30 kilometres away. (See later Posts on these types of stones and trace fossils.)
STONE 8
This green stone has also polished well, with only one small area that is not completely smooth.
There are a lot of green stones like this on southern beaches, some of them argillite (which I don’t think this one is), many with interesting patterns in them. They usually polish well.
STONE 9
After the initial tumble in 100 grit (Stage 2), I recorded that Stone 9 had a medium sized pit in its side. After the 220 grit tumble, the pit remained. I would normally have considered discarding the stone at this point, but for the sake of this series of Posts, following these 40 stones from rough to polish, I continued with it. After the 320 grit tumble, I noted there was some smoothing out of the pit, but it remained.
Stone 9 is a breccia, a type of stone where small angular fragments of (a diversity of) rocks are cemented together in a fine-grained matrix. As John Farndon puts it on page 95 of The Complete Illustrated Guide to Rocks of the World (2012), these sharp-edged fragments were caught up in the matrix before there was any time to round off the rough edges. The pit I had noted is in effect one of these fragments sunken below the surface of the stone, probably because it is softer and has worn away more quickly than the rest of the stone (similar to Stone 4, as noted in the previous Post). However, generally speaking, most of Stone 9 has polished well:
The main issue when polishing a breccia is whether all the various fragments will wear away at the same rate and whether they will all polish. The original pit is much shallower now but it is readily felt when a thumb is rubbed over it.
In the course of polishing, two other fragments in the breccia have failed to polish as a result of their relative softness:
STONE 10
This is another breccia, and it has polished quite well.
From the outset of tumbling, however, it has had a small but relatively deep hole in it, where maybe a small fragment has become dislodged. When inspecting it after the 320 grit tumble, I noticed that a couple of small indentations had also become apparent, probably where softer stone fragments had worn away a bit more quickly than the rest of the stone.
Breccia stones are always interesting, because of the diversity of colour and shape of the embedded fragments. Many of them can be found along the southern coast.
Stones 11 to 15 are dealt with in the next Post in this series.
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