Month: November 2023

An igneous stone, most likely rhyolite, this stone was found on Gemstone Beach or the coast just north of there along to the Waimeamea River mouth. I initially learned to recognise rhyolite stones at Slope Point and then, afterwards, started to notice them at Gemstone Beach as well. Stone of the Day #20 is a type of rhyolite that I now see each time I fossick at Gemstone Beach (there’s a very similar stone near the end of this Southern Sojourn Post).

Stone #20, Side A.

Stone #20, Side B.

It is not always possible to get a good smooth polished rhyolite stone but this one turned out quite good. The small light-coloured crystals have interesting shapes.

Rhyolite is a volcanic “felsic” rock, having a high silica and feldspar content. (“felsic” comes from “fel” from feldspar and “sic” from silica). “Felsic” rocks are not only silica rich but also tend to have a lot of aluminium, sodium and potassium, often giving them a lighter colour. Rhyolite is an unusual mineral as it can take quite different forms – University of Auckland Geology notes that it can be pumice, ignimbrite, obsidian or flow-banded etc., depending on whether the molten rock has erupted or flowed, what its gaseous content is, and how quickly it has cooled. Geologyscience.com notes that rhyolite is the extrusive form of granite, but that rhyolite has a fine-grained texture with phenocrysts, which are small crystals sometimes embedded within the rock (as apparent in Stone #20).

Go to Stone of the Day #21.

Go to the beginning of this Series. Go to the Series Index.

There are a range of green hues in Stone of the Day #19, from a very light green to the pistachio green associated with epidote to a dark gray-green. A number of very fine clear silica veins are also apparent in the close-up images.

Stone #19, Side A.

Stone #19, Side B.

Its size is 4.5 cm long and 3 cm high. Parts of the stone are still a bit rough and there’s a large chunk that has been knocked out of the stone during the polish tumble, noticeable at the bottom left of Side A. The darker patches in particular have not taken a polish very well.

Go to Stone of the Day #20.

Go to the beginning of this Stone of the Day Series. Go to the Series Index.

At first glance on the beach, Stone of the Day #18 looked like a rather ordinary white stone. I looked at it more closely and could see that it was probably a quartzite with a lot of detail in it. There is a lot of fragmentation, along with at least the appearance of fine veins.

Stone #18, Side A.

Stone #18, Side B.

This is one of my most favourite types of stones because of the minute detail, the floating cloud-like fragments, and that fact that it tumble-polishes well.

Go to Stone of the Day #19.

Go to Stone #1 in the Stone of the Day Series for November. Go to the Series Index.

It was its bright dark red veins that brought this stone to my attention when I came across it on Gemstone Beach. The veins are the same colour as areas in Stone of the Day #3 and Stone of the Day #10, probably a kind of iron oxide.

Stone #17, Side A.

Stone #17, Side B.

I am not sure what kind of stone this is. The close-ups also revealed to me that there are some tiny holes and even scratches on the surface of the polished stone. It has not taken a high polish but the tumbling process keeps the surface clear so that its colours are always on display.

Go to Stone of the Day #18.

Go to the beginning of this Series. Go to the Series Index.

Stone of the Day #16 appears to be a fine-grained mudstone, maybe argillite. It contains a variety of dark markings, either from darker sediment layers, disrupted and displaced by stresses, or from trace fossils such as tracks and burrows.

Stone #16, Side A.

Stone #16, Side B.

This is a larger stone than many of the previous Stones of the Day, being 7 cm by 5 cm.

I found this stone on Gemstone Beach on Friday 27 January 2023, quite early in my Southern Sojourn. Its discovery is mentioned in this Post from that time. I tumbled it in a medium 220 silicon carbide grit for about 10 days, then in a fine 400 grit and a finer 600 grit for similar periods. After nearly a couple of weeks tumbling in tin oxide polish powder, it finally had a burnishing tumble in borax for a few more days. After it had finished this tumble-polishing, I was surprised to see a scratch running across the top left-hand corner of Side A and wondered if this was caused during the tumbling. However, looking back at the photo I took of the stone when I found it on the beach, it appears the scratch was there originally. I probably did not seek to remove the scratch during the tumbling process as I wanted to preserve the dark markings.

Comparisons between the rough stone as found on the beach and the final polished stone are possible due to the photos I took then and now (though I have had to rotate Side A of the rough stone for comparisons to be made). These photos show how tumble-polishing has smoothed the stone, removing some holes, scratches and rough areas. The dark markings have sometimes changed as the surface layer was removed. First below are comparisons of Side A of the rough and polished stone. The rough stone images are on the top line of the following two sets of photos. The “Top of rough Side A” image shows the original long scratch, along with a few fingernail size indentations that were removed with tumbling (such indentations are also apparent in the “Bottom of rough Side A”):

Side A, rough stone when found.

Top of rough Side A.

Bottom of rough Side A.

Side A of polished stone.

Top of polished Side A.

Bottom of polished Side A.

Comparisons between the rough and polished versions of Side B again show how tumbling has smoothed out the stone but also changed some of the dark shapes (especially at lower right of image labelled “Bottom of rough Side B”, but also noticeable on the top part of the stone).

Side B, rough stone when found.

Top of rough Side B.

Bottom of rough Side B.

Side B of polished stone.

Top pf polished Side B.

Bottom of polished Side B.

Go to Stone of the Day #17.

Go to Stone #1 in the Stone of the Day Series for November.

Today’s Stone of the Day from Gemstone Beach contains veins, going in different directions, that speak of stages of displacement as various stresses have cracked it and pushed it around.

Stone #15, Side A.

Stone #15, Side B.

I found this stone on Gemstone Beach during my second-last Southern Sojourn fossick there, on Friday 28 April. It features as the third find in this Post – and it does not appear to have been changed much at all by tumbling.

Stone of the Day #15 is probably a form of rhyolite, an igneous rock. It has some similarities to ignimbrite stones found along this southern coast. It is the presence of tiny gaseous pockets (which look to have been infilled by some mineral) that tells us this is an igneous rock. Ignimbrite originates as the deposit of a pyroclastic flow, which is a hot suspension of particles and gases flowing explosively and rapidly from a volcano. New Zealand geologist Patrick Marshall originally came up with the term in the 1930s meaning “rain of fiery rock dust”. Some ignimbrite can be very loosely deposited, with lots of pumice in it. If it is buried at depth, it becomes compacted, gases and liquids are squeezed out, and it becomes solid and fine grained and glassy, like much of the material in Stone #15.

See this Post for more information on this kind of stone.

Go to Stone of the Day #16.

Go to the beginning of this Stone of the Day Series. Go to the Series Index.

At 3.75 cms long, this small stone has a lot going on. Side A especially has a lot of brecciation (fragmentation).

Stone #14, Side A.

Stone #14, Side B.

It has tumble-polished really well, being smooth and shiny.

Go to Stone of the Day #15.

Go to Stone #1 in the Stone of the Day Series for November.

This is another black and white stone, like Stone of the Day #12, but of quite a different composition. The close-ups reveal lots and lots of tiny white crystals. This means the stone is an igneous one, originating in molten lava, the crystals forming as the rock cooled and hardened.

Stone #13, Side A.

Stone #13, Side B.

This could be the same stone as Stone W44 in this Post about rough black and white stones from Gemstone Beach. There is an exploration of how these stones are produced, and the variety of types of them, following Stone W81 in the same Post.

This is a good example of a stone that I tend to treat as commonplace and ordinary, until I look more closely at it. It then turns out to be amazing.

Go to Stone of the Day #14.

Go to start of the Stone of the Day Series for November. Go to the Series Index.

The white “dots” in Stone #12 caught my eye when I spotted it on Gemstone Beach. Close inspection revealed well-defined rounded crystals – or are they worn fragments of quartz (making this a “conglomerate” stone)? This is sometimes a puzzle with stones that look like this – it can be hard to tell whether they contain crystals or fragments unless you have some appropriate geological knowledge.

Stone #12, Side A.

Stone #12, Side B.

My eye was initially drawn to the round white quartz in the stone. However, after looking more closely at the polished stone, I noticed crystals or fragments of something else there as well, having a rough silvery-gray appearance.

Conglomerates are similar to breccia in that they both consist of fragments of rocks cemented together in a fine-grained matrix. The difference between them is that the fragments making up conglomerates are rounded and worn while those making up breccia are angular and r0ugh. Geologyscience.com has an excellent page on conglomerates, including how and where they form.

Go to Stone of the Day #13.

Go to start of the Stone of the Day Series for November.