“Exercise Tiger” 75th Memorial on Slapton Sands

One of my favourite beaches is Slapton Sands in Devon, England, a pebble beach I visited in 2016 and 2017.

TumbleStone Blog has a series of Posts on this fascinating beach and its history and geography.  The first Post, dated 8 September 2016, is here. One of the Posts discusses the tragedy of Exercise Tiger, a practice for D-Day, with a planned landing on Slapton Sands. Slapton Sands had a number of similarities to Utah Beach, where US soldiers were to land. On 28 April 1944, in the bay off Slapton Sands, the convoy of landing craft carrying US troops was intercepted by German E-boats. Two of the landing craft were sunk and one badly damaged, with the loss of 749 of the US servicemen. Due to the secrecy surrounding D-Day and its preparations, the disaster was kept secret and often it was decades later before relatives learned what really happened to their fathers, brothers or sons.

To commemorate the 75th anniversary of this tragedy, an installation artist, Martin Barraud,  has laid 749 bootprints in the pebbles of Slapton Sands. 

Source: BBC 28 April 2019 http://www.bbc.com/news/uk-england-devon-48082397

The installation was unveiled by the Remembered charity and will raise money for veteran employment projects. An article in the Daily Mail on 28 April 2019 provides an excellent and well-illustrated account of Barraud’s project as well as the history of “Exercise Tiger”.

See also this short BBC video clip on the installation.

Martin Barraud is in the middle of an ambitious installation project in the UK called “There But Not There”. His aim is to make visible the dead of World War One, giving them a shape and a name. As an Evening Standard article puts it, “There are ghostly Tommies in St Pancras station, in a chapel in the Tower of London, in a football stadium: either a six-foot aluminium silhouette with head bowed, or a Perspex figure you see, then somehow don’t see, sitting down.” Barraud, has given every one of them the name of a real soldier.

Martin Barraud sitting amidst perspex figures of WWI soldiers. Source: Evening Standard http://www.standard.co.uk/go/london/arts/artist-martin-barraud-on-his-project-to-memorialise-fallen-first-world-war-soldiers-across-the-a3812606.html

By November this year, the centenary of the end of the war, he hopes to have sold a figure to represent every one of the 883,246 men from Britain and Ireland who died, the funds to go to military charities.

The Seven Stages in Tumble Polishing Stones: The End Result, Stones One to Five

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]:

1-5 rough
Stones 1 to 5, rough, as collected
Stones 1 to 5, after polishing


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 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 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.


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.


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.

The Seven Stages in Tumble Polishing Stones: Stage Seven, Borax Burnishing Tumble, 15 October to 25 October 2018

(This is the 11th Post in this series – the first Post can be found here.)

Steve Hart, in “Modern Rock Tumbling” (2008), attests that burnishing can make a “significant difference” in the appearance of polished stones (page 44). As one tumble-polisher has put it, “Sometimes stones are a little ‘hazy’ when they come out of the polish and small particles of polish are hidden in the tiny crevices of the stones. It helps to clean them up by burnishing the rocks in soapy water for 4-5 days.”

It was while I was doing my initial internet searches on how people tumble polished stones that I came across the idea of a post-polish burnishing tumble for up to a week using borax. Borax is sold as a white powder that dissolves easily in water. It has been used as a detergent, a food preservative (now banned for this use in some countries), an anti-fungal compound, a weed killer and a low-toxicity insecticide. It is used by tumble-polishers because it is a clean soap, lacking perfume and other additives (some use Sunlight or Ivory soap flakes instead). So I tried it and I thought that the results were great – polished stones seem to sparkle just a little more. Whether this is the case or not, I am not entirely sure, but I have the attitude that if it seems to work, why not continue it! In general, I think that a soap tumble of newly-polished stones for a day is the least that should be done, to remove polish particles and to give a final clean. My established routine practice is a tumble for around one week in one to two tablespoons of borax.

I polished the 40 Riverton stones, that this series is about, in borax for 10 days straight after the pro-polish tumble. This was a little longer than usual as I was busy with other things over this period. Firstly, the washed stones and pro-polish beads were replaced in the 4lb barrel, with water, and then about two tablespoons of borax were added.

001 borax
Two tablespoons of borax in the 4lb barrel in which the stones and plastic beads have been placed, topped by water.

At the end of the tumble, the stones were washed then given a final weighing:

The starting weight of the unpolished stones, right before I began to work with them, was 1605 grams, the end weight is 1244 grams. This means that 361 grams were worn away during the whole polishing process, which is 22.5%, just over one-fifth, of the stone material.


(Stage 1 = Stone collection)

Stage 2, 100 grit = 9 days 10 1/2 hours, followed by soap tumble of 12 hours

Stage 3, 220 grit = 9 days 2 hours, followed by soap tumble of 18 hours

Stage 4, 320 grit = 9 days 19 1/2 hours, followed by soap tumble of 3 days 1 1/2 hours

Stage 5, Pre-polish = 5 days 19 hours, followed by soap tumble of 20 hours

Stage 6, Pro-polish = 14 days 5 hours

Stage 7, Borax = 10 days 2 hours

This adds up to the stones spending 49 days and 8 hours tumbling in grit and polish, with an additional 15 days and 5 1/2 hours spent tumbling in soap and borax (a total of 64 days and 13 1/2 hours). Note that this is not the minimum time needed – I often extended tumbling by a day or two in some Stages, and there were occasions where tumbling in soap went on far longer than usual. Furthermore, a smooth beach stone can usually skip Stages One and Two.  

The following Posts in this series look at each of the polished stones in detail to assess the impact of this process on them. The first examines Stones 1 to 5. 

The Seven Stages in Tumble Polishing Stones: Stage Six, Pro-Polish Tumble, 1 October to 15 October 2018

(This is the tenth Post in this series – the first Post can be found here. Apologies for the delay that’s interrupted this series.)

In his book, “Modern Rock Tumbling” (2013), Steve Hart, a mechanical engineer, comments that how a stone becomes shiny after polishing is both a miracle and a mystery:

There is no one who really knows what happens inside the barrel when polish is added to the rocks. The Silicon Carbide stages were relatively easy to understand, because the process was primarily mechanical erosion or abrasion. Not so with polish. There are “mysteries” that don’t add up… There are theories that a given polish may liquefy the top layers of molecules on the rocks and rearrange them so the surface is smoother. Others say it may be an acid/base pH thing, or perhaps a frictional heat buildup phenomenon which causes the rock surface to flow… Another theory is that the rock surface is simply mechanically hammered… down to a smoother condition… After really trying to understand the polish process, I had to give up, because no one… really knows what’s going on (pages 40-41).

Stage Five of tumble polishing used a “pre-polish” tin oxide powder of five micron size. Stage Six uses a “pro-polish” tin oxide powder of one micron size. In general terms, I am following the instructions provided by Colin Simmons of the Rotorua Rock and Gemstone Shop from whom I bought my tumblers. He also sells the two grades of tin oxide powder. Colin recommends that the Pro-Polish tumble should be for at least seven days, and he states that the pro-polish mixture is retainable and reusable for about six batches of tumbling as long as it does not become contaminated. In this instance, I used a pro-polish mix that I had used five times before.

The procedure for starting the Pro-Polish tumble is the same as for the Pre-Polish stage, using the key ingredients: the 4lb tumbler, the white pro-polish tin oxide powder, a plastic shaker with a tight lid, and the plastic beads kept for pro-polish tumbling (see the relevant previous Post for details):

I try to tumble a pro-polish batch for at least 10 days, even though Colin Simmons states that it is okay to tumble for only seven days (as the minimum). In this case, I had taken on a temporary job which kept me busy while the stones were tumbling and I left them for a total of 14 days and five hours before taking the barrel off the machine. The process of taking the stones out of the barrel and washing them is the same as detailed for the Pre-Polish Stage:

I then use my pro-polish toothbrush to finish cleaning the barrel before the next stage:  

However, instead of then putting the stones in a soap tumble, they undergo a longer tumble wash in borax as a burnishing process, which is Stage Seven, described in the next Post in this series.

“The Secret Kissing of the Sun and Moon” by Hang Massive

A video of an instrumental piece, set in the grand geological landscape of Ladakh.

Wikipedia: Ladakh (“land of high passes”) is a region in the Indian state of Jammu and Kashmir that currently extends from the Siachen Glacier in the Karakoram range to the main Great Himalayas to the south, inhabited by people of Indo-Aryan and Tibetan descent. It is one of the most sparsely populated regions in Jammu and Kashmir and its culture and history are closely related to that of Tibet. Ladakh is renowned for its remote mountain beauty and culture.

Wikipedia: Danny Cudd (England) & Markus Johansson (Sweden) met in 2010 in India and formed Hang Massive soon after. They were one of the first acts to become known for playing the Hang, a new-age percussion instrument, and quickly gained followers for their music videos, becoming “the epitome of the Hang”.


Gemstone Beach Gems Off to Germany

I mentioned towards the end of a previous post that I had met Maike and Martin, a young couple from Germany, on Gemstone Beach in March. I agreed to tumble-polish some stones for them. Today I finished the process, and Petra my wife will take the stones to Germany when she travels there in a couple of days. 

Altogether, Maike and Martin collected 83 stones on Gemstone Beach. The largest is  6 cms by 3 cms, the smallest being about half the size of my small finger-nail. All 83 fitted nicely into a 3lb tumbling barrel. I am very impressed with the resulting product, an excellent set of polished Gemstone Beach “gems”! Maike and Martin collected well.

The following are photos of the original stones, dry, then how they looked after their first tumble in 320 grit, and then how they looked after their final polish: 

Firstly, the stones were tumbled in 320 grade silicon carbide for seven days, then tumbled in Sunlight soap for six hours. Secondly, they were in a five-micron tin oxide “pre-polish” tumble for five days, then tumbled in Sunlight soap for 16 hours. Thirdly, a one-micron tin-oxide “pro-polish” stage took eight days, followed by 27 hours in borax soap. The whole process took 22 days. This is probably the shortest time I have ever polished stones, but the outcome was very good. Initially the stones weighed 780 grams, finishing up at 689 grams, thus losing 10% of their mass, most of it in the 320 tumble stage. 

The following photographs provide comparisons of some of the polished stones with how they looked initially, when found on Gemstone Beach. To start off with, here are three of the larger ones:

Further comparisons for larger stones:

Some of the medium-sized stones:

Smaller stones:

One of the stones is a small jasper button:

Finally, some of the smallest stones collected by Maike and Martin:

When packaging the stones to send to Germany, to complete Maike and Martin’s set of outstanding Gemstone Beach stones, I added a hydrogrossular garnet and a stone with a fossil worm cast. These are iconic Gemstone Beach gems, with the hydrogrossular garnet especially sought after, and the fossil worm cast being reasonably distinctive to this coast. As they were absent from Maike and Martin’s haul, I took the liberty of adding them to the package heading overseas.

White hydrogrossular garnet (left) and what is probably an argillite stone with a fossil worm cast (right).


March 2019 Stone Collecting Trip to Southern New Zealand – Gemstone Beach, from the Car Park to the Waimeamea River Mouth

Gemstone Beach, near Orepuki on the south coast, “is in a constant state of change with the surface changing from sand to stones with the storms and tides” (Southland website). This year, I spent three weeks in March in Southland, collecting beach stones, and I made five visits to Gemstone Beach.  This year the beach offered lots and lots of good stones, maybe the most I have seen. As a result, I collected probably about 18 kilograms of stones from there to take home to polish. These included over 100 stones with fossil worm casts.

Gemstone Beach is just a kilometre from Orepuki, a small village on a sparsely settled part of the southern coast of New Zealand. Gold was mined in the district during the last two decades of the nineteenth century, and in the 1880s the southernmost Chinese settlement in the world was to be found there. It is said that semi-precious gems such as garnet, sapphire, jasper, quartz and nephrite can be found on Gemstone Beach, although some of these are very rare. Quartz and jasper are reasonably common, and you can sometimes find hydrogrossular garnet stones and argillite stones containing fossil worm casts.

Orepuki location
Location of Orepuki and Gemstone Beach on the Southland coast


When you initially walk down from the car park and emerge onto Gemstone Beach (see map below), it can be a real disappointment if you’re expecting to see a lot of great stones. The immediate area is usually primarily sand, with only a handful of widely-spaced stones around. Just to the left is a small (un-named) stream which many people go to in order to see if there are interesting stones there but it also usually disappoints.

Gemstone Beach entrance mapcarta
Entrance area of Gemstone Beach – from Mapcarta

This photo (below), taken by me in February 2018, shows a family on the beach just a few metres away from the car park. Notice the wide scattering of stones – usually there are less than 10 per cent of this number in this area. The two people in the background are looking towards the un-named stream.

feb 2018

When you come onto the beach from the car park, about 200 metres to your right (in a north-west direction) is a stream that flows across the beach to the sea, Taunoa Stream (see map above). At times the depth of the stream water can be high enough to dissuade anyone wearing shoes from crossing, unless they take their shoes off. However, it is further along the beach beyond this stream that the stones will be found. At high tide, it can be impossible to ford the Taunoa Stream because of the waves rushing in to the foot of the nearby cliff. This year, the stream bed was full of stones, the water having swept the covering of sand from them. A significant part of my second visit to Gemstone Beach this month (March) was spent examining the stones in the stream.   


At times, one has to walk a long way past the Taunoa Stream to find stones (see photo below, left). This year, after only about 100 metres past the stream, large drifts of stones covered the beach (below, right). 

And the stones were scattered right up to the cliff face, unlike at other times when I have visited.

Each drift of stones on the beach consisted of a good layer of pebbles, some on the small side but many of a good polish-able size.


About one kilometre along the beach from the Taunoa Stream is the mouth of the small Waimeamea River.

Currently – and this was also the case when I was there in September 2018 – there is a large bank of stones along the beach, starting about 400 metres before the Waimeamea River mouth and continuing as far along the beach as I could see (some kilometres). The bank is quite high, maybe four or five metres in places, and up to 50 metres wide. Some of the stones here are bigger than those closer to Taunoa Stream, too big for my tumble polish barrels.

The stone bank holds back the waters of the river, preventing them from reaching the sea, creating a lagoon running parallel to the coast. However, water seeps under the stones to meet the incoming waves, the stones start to collapse, and eventually a channel appears for the river to flow out, as I had observed in September 2018.


Birds are perhaps the most common wildlife encountered on southern beaches. At the mouth of the Waimeamea River, I walked past some seagulls and terns, trying hard not to disturb them:

Further back down the beach, near the Taunoa Stream, in my second encounter, I met up with Maike and Martin, a young couple from Germany on holiday in the South Island. They were picking up interesting stones and asked me how they could get them polished. I volunteered to do a batch for them. I also gave them a short break from living out of their car, inviting them back to the crib at Riverton. Below are some of the stones I am in the process of polishing for them.

See this Post to see how these stones looked after polishing.

My third encounter, from a distance, was with some horse riders, glimpsed as I left Gemstone Beach for the last time during this series of visits. They were moving towards the car park from the south-east of the beach, from the direction of Monkey Island. There were four riders on horses, one horse carrying a child being led, and a wagon drawn by two horses.

See the later Post Some “Gems” from Gemstone Beach to see how a batch of 95 stones I collected turned out after polishing.

TumbleStone Calendar 2019 – October, November and December

In October (above, left), the lone figure of my friend Ray walks ahead on Gemstone Beach as I take about five steps every five minutes. I am searching for the elusive hydrogrossulars, so shiny that tumble polishing usually does not improve them. November (above, middle) also features rocks that shine but this time it’s the large mica flakes in them that sparkle. Found at Joyce Bay on the West Coast of the South Island, one such rock sits in my lounge room, beside the fire. Finally, in December (above, right), we return again to Riverton, my favourite set of beaches where I have found some of my favourite stones.

Also see TumbleStone Calendar 2019, TumbleStone Calendar 2019 – February, March, April and May and TumbleStone Calendar 2019 – June, July, August and September.


TumbleStone Calendar 2019 – June, July, August and September

The June page for this calendar (above, left) has a photo of Kiritehere Beach, on the west coast of the North Island, which Petra and I visited in September 2018, and some of the rocks with monotis fossils that we found there. The rocks on this beach are full of these fossils. I have not tried to tumble polish them. Budleigh Salterton features on the July page (above, right). This village in Devon has a pebble beach full of red iron-stained quartzite stones from the cliffs nearby. We have visited there when we have been in Devon in the past three years. For more detail on Budleigh Salterton and its stones, see the comments on Stone #7 in the post on Twelve Stones, Part Three.

For August, one of my favourite stones appears, banded rhyolite. Stones of this type can be found on beaches along part of the south coast of the South Island, especially around Riverton and Orepuki. The beach featured on the August page is the beach past the Back Beach at Riverton (just beyond the end of the road). September’s beach is another one in the UK that Petra and I visited in 2018, Penmon Point on the Isle of Anglesey in Wales. There I found a black stone with interesting patterned fossils in it (I have not been able to identify the fossils), and a number of limestone pebbles also with fossil shells that are much fainter.

Also see TumbleStone Calendar 2019TumbleStone Calendar 2019 – February, March, April and May and TumbleStone Calendar 2019 – October, November and December.