This Series of Posts provides an introduction to Gemstone Beach (Orepuki, Southland, New Zealand) and a basic guide to many of the stones commonly found on that beach. Previous Posts = Part One: Location and Carpark; Part Two: Main Features of the Beach; Part Three: Sources of Information & What is a Gemstone; Part Four: Brief Introduction to Geological Terms for Stones; Part Five: Translucent Stones; Part Six-A: Stones that are Predominantly White; Part Six-B: Stones with White Spots & Crystals; Part Six-C: Stones with Opaque White Veins & Bands.
The previous three Posts looked at seven types of white stones, or white parts of stones, that can be found on Gemstone Beach: 1 – White Milky Quartz Stones; 2 – White Hydrogrossular Garnets; 3 – White in Other Coloured Hydrogrossular Garnets; 4 – Stones with Opaque White Spots; 5 – Stones with Opaque White Crystals; 6 – Stones with Opaque White Veins; and 7 – Stones with Opaque White Bands. This Post looks at 8 – Stones with Opaque White “Patches”. There could be other types of white stones on Gemstone Beach but these are the ones I am currently aware of that are most likely to be found.
8 – STONES WITH OPAQUE WHITE “PATCHES“
This section contains a wide range of stones with opaque white material in them. Some have white patches in a conventional sense, others have a variety of white material and shapes. I have struggled to organise the stones in some sort of logical sequence or set of categories, so they are set out in a rather haphazard way. I also don’t always know the identity of a mineral or stone. What I hope this section will do is encourage you to look more closely at a stone you pick up on Gemstone Beach, to see how it is made up of a variety of colours, patterns and minerals.
Sometimes a stone looks like it has a little white in it but closer inspection suggests that the whole stone is likely to be white originally but then it has been “flooded” with a colourful mineral; or maybe the white is the matrix for fragments of a colourful stone (a breccia):
As mentioned in the previous Post, a “breccia” stone (like Stone W118) consists of small angular fragments (clasts) of one or more rocks cemented together in a fine-grained matrix. The small fragments come from some form of erosion and their angular shape means they have not “travelled” far, they have not had the chance to be smoothed. A similar type of stone with rounded smooth fragments is called a “conglomerate”. A number of the stones shown below are also likely to be breccia.
Stones W121 and W122 below are white stones with darker and/or colourful areas – the former at first glance looks as if some small areas of minerals are in it but closer inspection suggests it may be, like Stone W122, a breccia stone with opaque white quartz being the matrix:
The next two stones have black and white “patches” but the shapes and mix of the contrasting materials are very different – both have very interesting detail:
Another two breccia, both with white fragments (clasts) but one with a light green-coloured matrix, the other with a brown matrix:
Some of the stones I have found on Gemstone Beach have “hints” of white and closer inspection is needed to see their patterns more clearly:
Maybe Stones W127 and W129 (above) are breccia and Stone W128 is quartz “flooded” with dark red iron oxide. Stones W130 and W131 (below) have “wedges” of white material but their details are very different – Stone W131 has a very interesting margin along the edge of the white:
Below is a selection of five other stones with white “patches” in them, again illustrating a diversity of colours and patterns:
The different ways that black and white “patches” can occur are well illustrated by the following three stones – Stones W137 and W138 are breccia but with contrasting colours of fragments, while Stone W139 appears to be a white quartz stone with black mineral inclusions:
The next four stones have small white “patches” that are part of unusual structures. Stone W140 has an unusual light green colour (the matrix), probably caused by the mineral epidote, with small white fragments (quartz?) – it is an example of a breccia where the matrix makes up more of the stone than the fragments or clasts. Stone W141 could also be a breccia but with the epidote material being the fragments – I am not sure. Stone W142, most likely a quartzite, has been subjected to stress and veins filled in the resulting gaps. Stone W143 is a green stone with gorgeous small orbs in it, along with small patches of white and transparent quartz – I’ve always imagined such orbs are created in heat flows within a cooling rock, but I also know that sometimes they arise as crystals grow within such rock.
The next two stones both have what at first sight appears to be small lone bits of white. However, closer inspection reveals that Stone W144 is probably a quartz stone flooded by a dark red mineral (maybe iron oxide). And Stone W145 may be a quartz breccia(?) with a singular opaque white clast – it is tempting to call it a “xenolith” (a “foreign” stone in a stone of different material) but this term is usually used in reference to igneous rocks.
The final two stones with white “patches” are both green quartzites with small areas of opaque white material. This white material seems different from the white quartz seen in other stones, and may be a different mineral.
Both stones also have a number of thin veins, mainly transparent.
The next Posts in this Series (not yet all available) will look at green Gemstone Beach stones. The first one, just completed, is on green argillite stones.
DISCLAIMER: Over the past six years I have visited Gemstone Beach more than 150 times, also walking further along the Te Waewae Bay coast. But I am not a geologist – I am an amateur beach stone collector and polisher. I have done some reading and research about different types of stones, but my knowledge is limited and open to correction. Some of the photos used in these Posts were taken at Gemstone Beach in April 2023; some come from my fossicking trips there over the past three or four years; and a few were taken after I looked through my “finds” in my stone shed at home. All the stones are “rough”, not polished. The coverage of stones is not comprehensive and is biased by my own interests and what catches my attention. The photos are not perfect, often having some reflections in them. But I hope these Posts prove useful to the “passing motorist” and others who find themselves on this interesting beach.
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