In this Series of Posts, 30 recently polished stones are examined – they were found on the coastline that runs north from Gemstone Beach near Orepuki in Southland, New Zealand. Many interesting and beautiful stones can be found here, brought down from the mountains of Fiordland by the Waiau River. This Series also illustrates some of the challenges a tumble polisher faces to obtain a completely smooth and well-polished stone. Part 1 in this Series of Posts described Stones 1 to 5, with the following Parts dealing with another five stones each. The five stones featured here are smaller than the first 20 and getting good clear photos of them can sometimes be more difficult as a result.
Stone 21 is a light green argillite with tiny lighter-coloured trace fossils in it. Locals often refer to these as “fossil worm cast” stones.
This Post provide some details on argillite, a hardened mudstone. Argillite stones with trace fossils in them are reasonably common on and near Gemstone Beach (and can be found elsewhere in New Zealand as well). Trace fossils are not fossils of an animal itself but of something they have left behind them, like burrows or tracks. Stones Gn40 to Gn62 in this Post are interesting examples of trace fossil argillite stones and their diversity. A small trace fossil stone is in the TumbleStone Hall of Fame. Returning to Stone 21, the bottom half of the stone has markings on it that may not be trace fossils in the usual sense but be ripples in the sediment from sea currents or from a fish swimming by.
The process of tumble-polishing does not always lead to argillite stones with a high polish. However, it removes the dull weathered surface so that things like trace fossils can be clearly seen, even when the stone is dry. I tend to tumble trace fossil stones in 400 grit, followed occasionally with a 600 grit tumble, then the polish stage. Skipping the use of the coarser grits helps to preserve the trace fossils as they appear on the surface, though some small holes and scratches may remain.
Stone 22 is a small jasper (I think) of lively brown colour, with an interesting milky quartz vein and a few very thin transparent veins.
There appear to be inclusions of quartz and maybe mica which interrupt a smooth surface. The largest vein also has some holes in it, as can be seen especially on Side A. To remove these using tumbling would have made the stone very small – it is currently just under 2 cms at its widest – with no guarantee that all the “imperfections” would be absent.
A remarkable little pebble, Stone 23 contains a small vein with tiny brecciations filling it. It’s like a glimpse into a hidden shrunken world.
As noted in relation to Stone 1, breccia are angular fragments of stone that have been compressed together in a fine-grained matrix. There are a number of different causes for brecciation, such as the accumulation of rock debris from weathering or erosion and the fragmentation of rock in a contact area between two faults (see GeologyScience.com). The brecciation in Stone 23 could have been caused by pressure stresses in the rock, pushing and pulling resulting in fragmentation then the creation of a vein into which maybe quartz has filtered, cementing the fragments. Again, as is illustrated clearly on Side B, the amount of tumbling necessary to produce a smooth surface would have resulted in a very small stone. Limiting the use of coarse grit means that a small scratch remains, marring the end product, but we have a fascinating little stone to admire.
Stone 24 is a small milky quartz.
The B side shows the result of an impact on the stone. This type of line often goes deep into a stone and so cannot be removed by tumbling without destroying the stone itself. Wikipedia notes that milky quartz is a very common variety of crystalline quartz. “The white color is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation.” Stones W1 to W3 in this Post are other specimens of milky quartz from Gemstone Beach, in the rough unpolished state (but still very smooth). This type of quartz stone tumble polishes very well.
Stone 25 has distinct ribbons of green that caught my eye on the beach (apologies that the photos are not sharper).
The banding in this stone is a mix of mudstone (argillite) and sandstone (including some tiny quartz grains), very similar to Stone Gn14 in this Post. In such cases, I usually make sure that I do a 600 grit tumble before putting the stone to polish, to help the coarser material to polish better.
The final Post in this Series (not yet published) deals with Stones 26 to 30.
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