Thirty Gemstone Beach Pebbles for Doug: Part Two – Stones 6 to 11

Doug is the journalist for Whanganui’s River City Press, a weekly community newspaper. He interviewed me about collecting stones (the article is titled “John Paterson’s hunt for coloured stones”), and afterwards asked to buy some of my polished stones. I picked out 30 Gemstone Beach stones for him. Part One in this Series introduced Southland’s Gemstone Beach and looked at the first five stones. The next six stones are featured in this Post.

Stone #6 is the same rock type as Stone #3, an argillite mudstone:

However, whereas Stone #3 is banded and a strong green colour, Stone #6 is brecciated (fragmented) and a pale grey/green. There are two different kinds of stones that consist of cemented together fragments, breccia and conglomerate. “Breccia” (pronounced “bretch-ee-ah”) comes from the Italian for “rubble”. In a “breccia” stone, the fragments that make up the stone have not travelled far before being cemented together and so they are angular and sharp. In a “conglomerate”, the fragments have undergone some rounding from the travel they have experienced, usually from water, prior to being cemented together. Stone #6 is a breccia because its fragments are angular. Three of the main types of breccia are named after the processes that caused them – tectonic, sedimentary and igneous. “Tectonic” breccia is where massive stresses on a rock tears it into fragments which are then re-cemented together in a fine-grained matrix. “Sedimentary” breccia arise when fragments from different stones have been briefly transported by water or submarine debris flows and laid down as sediment prior to becoming a new rock. “Igneous” breccia can be caused by volcanic eruptions and the accumulation of small bits of the erupted material. A breccia where the fragments represent more than one rock type is called “polymictic” while one where the fragments come from just one rock type is termed “monomictic”. Stone #6 is a monomictic tectonic breccia of argillite. In other words, the original argillite rock has been torn into fragments by pressure and movement while underground. For more about breccia, see University of Auckland Geology and “January 2022, Stone of the Day #2 – Breccia and the Giant Landslides of Fiordland“.

Stone #7 is another argillite, a different kind of banded argillite from Stone #3:

In Stone #3, thin dark green bands or veins are present in a green stone. In Stone #7, there are a number of thicker bands of grey, brown and black that make up the whole stone. In this type of banded argillite, it appears that some extra heat and pressure has been applied, as in places the bands kind of melt into one another. This is the most well-known variety of Gemstone Beach’s banded argillite, a similar one being in this Post.

Stone #8 is ignimbrite, a type of rhyolite:

Rhyolite is an igneous rock with a very high silica content. It is the volcanic equivalent of granite, granite bring formed underground. Depending on the nature of its emergence, rhyolite can also take the form of obsidian, pumice or ignimbrite. In certain situations, extremely porous (full of tiny holes) rhyolite lava flows occur which then collapse into a massy glassy material, obsidian, a dark coloured volcanic glass. If rhyolite is part of an explosive eruption, it can form the sponge-like pumice, a very lightweight, light-coloured rock full of tiny air holes – the only rock that floats. Ignimbrite forms from hot, dense mixtures of ash and pumice ejected from explosive volcanic eruptions. New Zealand geologist Patrick Marshall originally came up with the term “ignimbrite” in the 1930s meaning “rain of fiery rock dust” (from the Latin “igni” for fire and “imbri” for rain). The volcanic ash layers cool and compact under their own weight, becoming solid rock. I come across quite a few stones like Stone #8 along the south coast during my stone collecting trips and I love their banding and colour. They conjure up for me images of deep space and galaxies and stars, and the rings of planet Saturn. Many of them tumble-polish well, though their tiny gaseous pockets can sometimes interrupt a smooth surface. See the Post “November 2023 Polished Stones: Stone of the Day #22” which reports on how I came to identify such stones as ignimbrite.

Stone #9 is probably a quartzite with a fine lace-like tracery of yellow mineral through it:

When quartz rock is worn down to very small grains of sand, it can then be subject to heat and pressure. The grains become cemented tightly together and form quartzite. During that process, other minerals may enter the scene and provide a colour for the complete rock or leave lines as are seen in Stone #9. Some more information on quartzite can be found in the entry for Wednesday 26 May 2021 in the Post “FB Group Posts: 26, 27 & 28 May 2021 – Kakanui Quartzites and Slope Point & Gemstone Beach Stones”.

Stone #10 is one of three trace fossil stones in Doug’s 30, the others being #14 and #20:

I found Stone #10 on Gemstone Beach in September 2021 – it is the third stone in this Post. Trace fossils are not fossils of an animal but are fossils of the traces they leave behind. “Trace fossils are what is left of the activity of some ancient critter”, writes retired kiwi geologist Brian Ricketts in his blog “Geological Digressions”. That activity includes burrowing, moving, excreting, fighting and feeding. Stone #10 is likely to be an in-filled trail or burrow. The material making up the trace is slightly harder than the rest of the stone and is wearing away more slowly, causing it to sit higher. The stone is argillite, a hardened mudstone, the same as Stones #3 and #7. It was laid down in the Permian Era, 250 to 300 million years ago. I have long been fascinated by trace fossil stones. There are many to be found on Gemstone Beach, where they are known locally as “fossil worm cast” stones. I have collected a wide variety of them – see Stones Gn40 to Gn62 in “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Seven-A, Green Argillite Stones”. They are the stones I have done most research on and many Posts in my Blog deal with them – see for example “The Fossilised Worm Cast Stones of Gemstone Beach and Riverton – Part Five: ‘Burrowing Worms’ of the Permian in Brook Street Terrane Rocks East of Tihaka Beach”. Brian Ricketts provides an excellent general introduction to trace fossils here. 

Stone #11 looks similar to Stones #1 and #2 which are black igneous stones with white crystals:

However, the close-up photos reveal a much less fine-grained matrix. There’s a lot of white quartz in there, maybe some silvery mica, and some black mineral like maybe hornblende. My uncertainty is because it is often difficult to identify a stone. Geology textbooks and online sites usually refer to much larger rock strata and their characteristics in general, or to a sole specimen of a rock type. There is a great diversity of stones on Gemstone Beach, including a wide set of gradations within each type. I pick up stones because of their colours and patterns (like those with interesting black and white spots, as in Stone #11), and the likelihood they will tumble-polish well. They also have to be small enough to fit into my tumbler barrels. While I undertake research to try to identify many of my stones, it is not always easy or successful. So I don’t really know what kind of rock makes up Stone #11. I just know I like the look of it.

The next Post in this Series looks at Stones #12 to 17 of Doug’s 30.