Thirty Gemstone Beach Pebbles for Doug: Part Three – Stones 12 to 17

Doug from Whanganui’s River City Press recently interviewed me about collecting stones for his article  “John Paterson’s hunt for coloured stones”. Afterwards he asked me if he could buy some of my polished stones so 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. Part Two featured the next six stones.

The first stone in today’s Post, Stone #12, is one of eight argillite stones in Doug’s 30, the others being Stones #3, #6, #7, #10, #14, #19 and #20:

Stone #12 is very fine grained and smooth. I used to think that it consisted of a dark layer and a light layer, with one of the layers eroding unevenly, revealing part of the layer beneath. However, I now think the two contrasting materials are mixed in together right through the stone, giving rise to its unusual camouflage-like or maybe turtle-back pattern. Argillite is a sedimentary stone. The three main types of rock are sedimentary, igneous and metamorphic. Sedimentary rocks are formed by the accumulation or deposition of mineral or organic particles on the surface of the Earth. They are thus formed from the weathering of pre-existing rocks and/or the pieces of once-living organisms such as shells. Layers of sediment accumulate and are then buried, compacted, and subjected to pressure and hot temperatures, up to 40 degrees Centigrade. You can often see a form of the original layers in the rock. Sedimentary rocks have not been heated enough to melt, and often can contain fossils (such as the trace fossils seen in Stones #10, #14 and #20). Most sedimentary rocks are classified according to the size of their grains or fragments. Siltstones and mudstones (like argillite) have grains that are not visible to the human eye and are very smooth. Sandstones have tiny grains that the human eye can usually distinguish and they sometimes feel slightly rough. Breccia and conglomerates have slightly larger fragments (see, for example, Stones #6 and #26).

Stone #13 is the type of stone I love to find on Gemstone Beach – it has tiny details of shape and colour that can only fully be appreciated in the close-up photos:

There’s lots of greys, oranges and whites in there, maybe feldspar and quartz, among other minerals. The dabs of pinky orange in Stone #13, a salmon pink to my eye, is a kind of feldspar. Feldspar is the most abundant mineral group in the Earth’s crust and contains a great variety of types. As Alex Strekeisen notes, there are more feldspars (60%) than all the other minerals combined in the outer crust. Plagioclase feldspar is one of the two main types. It is white and when in a rock it often looks like quartz. It weathers down to clay relatively easily. It is also commonly found in lunar rocks. Orthoclase feldspar is a potassium (alkali) feldspar which includes the orange/pink variety found in Stone #13, as well as in many granites.

Stone #14 is a trace fossil argillite stone:

My comments on Stone #10 provide some details about trace fossils and their argillite host rock. That stone, #10, is a grey/green colour, with the trace being thickish and an off-white colour. By contrast, Stone #14 is a much darker green and the trace on side A is more narrow, though longer, and an even darker colour. There are also traces on side B, less well-defined but of a similar colour. The two stones are a good illustration of the variety of trace fossil argillite stones. In July 2020, I produced a small photo-book which included a page (page 6) showing eight different specimens. While preparing the photos for Stone #14, I noticed some faint markings on both sides – these could also be trace fossils. Or they could be disturbances in the original sediments caused by water currents.

Stone #15 is one of those stones that becomes more fascinating the longer you look at it:

It is apparent that there’s a lot of little bits in the stone. The close-up photos reveal better detail of what’s there. There seems to be lots of tiny bits or crystals of quartz. Some of them could be transparent or translucent, some of them opaque. This relates to the three main ways of classifying how light passes through a stone. The first type is “transparent”, meaning light passes through the stone easily, as through clear glass or a clear quartz crystal; “translucent” means only limited light is able to pass through the stone, so that an object held behind it would look fuzzy; and the third category “opaque” means light does not pass through the stone at all (most stones are opaque). An interesting web page on the three categories provides the following useful illustrations:

Stones #16 and #17 are hydrogrossular garnets, Stone #16 being tumble-polished and Stone #17 being unpolished:

Gemstone Beach is especially well-known in connection with the hydrogrossular garnets that can be found there. As I note in a detailed Post on them, unlike the garnets we know as precious gems, hydrogrossular garnets come from dense rock masses, not crystals. Technically, hydrogrossular garnets are a calcium aluminium garnet with hydroxide partially replacing the silica found in other garnets. The first ever identification of hydrogrossular garnet in the world was in 1943 by Colin Hutton, from stones found in Nelson, and it is the most widely spread of the 13 minerals first described from New Zealand. More details can be found in “First Identified in New Zealand in 1943 – Revisiting the Hydrogrossular Garnets of Gemstone Beach, February 2023”. Hydrogrossular stones often feel waxy, and they have a slightly duller look, although when wet they shine brightly on the beach. Some are at least partly transparent. “Grossular” derives from the Latin word for gooseberry, referring to the light-green colour of some hydrogrossular garnets – Stone #17 is an excellent example. However, other minerals get mixed in, and white and brown are two other common colours of hydrogrossular stones. Hydrogrossular stones are easily polished and make excellent pendants.

The next Post in this Series looks at Stones #18 to #23 of Doug’s 30.