Thirty Gemstone Beach Pebbles for Doug: Part Four – Stones 18 to 23

I was recently interviewed about stone collecting by Doug, a journalist for Whanganui’s River City Press (here is the article). Afterwards he asked to buy some of my polished stones so I picked out 30 Gemstone Beach stones for him. In this Series of Posts, I present photos of each stone and provide comments on it. Part One in this Series is here.

Stone #18 has a deep orange colour, probably as a result of iron oxide:

The stone appears to have a lot of quartz in it. On especially side B, there are areas of opaque white quartz that contribute to its interesting character. Stones like these catch my eye on Gemstone Beach, and their high quartz content means that they will tumble polish well.

Stone #19 is a breccia, as was Stone #6:

The angular fragments in it indicate that they have not travelled far before being cemented together in the light-coloured quartz. Similar to Stone #6, it is likely to be “monomictic”, its fragments all coming from the one source. And it was probably a sedimentary rock originally. However, the fragments are not as densely packed as those in Stone #6, indicating they could have been floating in solution before being fixed in quartz.

Stone #20 is the third of the trace fossils in argillite, the other two being Stones #10 and #14. All three stones contain traces that belong to the ichnogenus Protovirgularia:

A trace fossil is technically called an “ichnofossil”. Such fossils are identified by their shapes, not by what made the shapes as this may not always be known. Furthermore, different animals could leave the same shaped trace. The term “ichnofossil” is derived from the Greek word “ichnites” meaning “footprint”. An “ichnogenus” is a group of trace fossils with similar characteristics. Ichnogenus Protovirgularia are trace fossil shapes consisting of “a small keel-like trail which is composed of an elevated median line and lateral wedge-shaped appendages alternating on both sides”, including lines of chevron shapes as seen in Stone #20. They were given this name in 1850 by Frederick McCoy, an influential Irish palaeontologist, who believed he was seeing the actual fossil of an ancestor of Virgularia mirabilis, the slender sea-pen, hence the term “proto-virgularia”. It was not until 1958 that the shape was identified instead as a trace fossil and one which was made by quite different animals. More information, with diagrams, can be found in my Post “I is for Ichnogenus Protovirgularia”.

Stone #21 is probably a quartzite full of the green mineral epidote:

Tiny bits of transparent quartz and white feldspar can be seen in this stone. But it has been flooded with an intense green mineral. Quite a range of green hues can be found in Gemstone Beach stones, and often the colour is due to the presence of epidote. Geologists often state that epidote is characteristically “pistachio green” – and Stone #6 comes close to that. The argillite of Stone #14 has also been coloured by epidote, the result being a much darker hue. However, epidote can also be responsible for the brighter more intense green as seen in Stone #21. For more on the colour green and the minerals that cause green in stones, see this Post.

Stone #22 is probably a small granite pebble, granite being made up of three minerals:

For geologists, a “mineral” is a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form (Wikipedia). Another definition is that a mineral is a naturally occurring, inorganic, solid, crystalline substance which has a fixed structure and a defined chemical composition (University of Auckland Geology). A mineral may occur as a large crystal, or as pieces between one millimetre to one centimetre in size, or as tiny microscopic grains. Minerals are the building blocks of “rocks”. Not many rocks are made up of only one mineral (one commonly-occurring example is quartz), with most rocks consisting of two or more. An often used example of a multi-mineral rock is granite, which is usually said to consist of three main minerals: feldspar, quartz, and mica. However, there are different types of feldspar and mica, and a number of variations of granite depending on the mix of minerals. The feldspar in Stone #22 is orthoclase, the pink/orange variety (see the comments on Stone #13) – this gives rise to what is called down south “pink granite”. If the feldspar is plagioclase, the resultant granite is predominantly white. In Stone #22, the orthoclase feldspar dominates even the quartz, giving it an orange hue. Mica is characterized by a layered, platy structure, consisting of thin, flexible sheets. It is common in a wide range of rocks, often appearing as tiny flakes. I have written about mica in “M is for Muscovite Mica from Joyce Bay”.

Stone #23 is known locally as a volcanic bubble-infilled stone, its more technical name being an amygdaloidal stone:

Molten magma often contains dissolved gas which can form bubbles in the rock as the pressure is released on eruption. These bubbles can get trapped in the solidified rock, forming tiny holes (called “vesicles”). These holes then fill with mineral-rich fluids which leave behind deposits of minerals such as quartz, chalcedony (agate), calcite and zeolites (zeolites are a group of minerals with a crystalline structure made up of silicon, aluminum, and oxygen). The resultant “spots” are called “amygdales” (also “amygdules”), a term that comes from the Latin and Greek words for almond, reflecting the almond-shape of many such in-filled vesicles. However, amygdales can also be round and some can be irregular shapes, as can be seen in Stone #23. Many of the dark-coloured stones with amygdales on Gemstone Beach are basalt.  

The next Post in this Series looks at the final seven of Doug’s 30.