I met Michelle at the end of a Gemstone Beach fossick in mid-September. She had a bag of stones she had just collected and we chatted for a while. She is definitely a stone enthusiast and had visited the beach many times. Her stones are important to her. I offered to tumble polish some of her finds. It is rare for me to make such an offer – often people collect stones that are not the best for tumbling -but Michelle’s enthusiasm runs deep and I knew she would appreciate the end product. She later brought quite a few of her stones around to where I was staying so I could pick through them to select the best ones for tumble polishing in terms of size, hardness and suitability. The first batch has now finished being polished, a total of 51 stones, with a few more still in the process, needing extra tumbling.
I have selected 24 of these polished stones for individual photos. This Post features 12 of them (see photo immediately above, top row, at the right) with a following Post containing the remaining 12. Stone 1 is a small thin stone, just two centimetres long. Its grey and black and small flashes of white make it quite attractive, and it has come out very smooth and very shiny. It is possibly a mudstone and is brecciated, meaning that tectonic stress has fragmented it in its history.
Stone 2 chose its own place in the sequence of stones – one side looks like it has the number 2 on it!
Stone 2 is a bigger stone, three and a half centimetres long. Most of it is black, maybe of volcanic origin, with a large vein running through it. Again, the vein will have been created by tectonic stress, and there is some brecciation present (the black fragments that can be seen). The vein that opened up is filled with colourful minerals, including quartz. This stone has again tumble polished quite well, with just a small part of the second side feeling slightly rough.
Stone 3 is an igneous porphyry with a very slight blue tinge. I have found a few of these myself on Gemstone Beach and elsewhere along the Southland coast. They are an attractive stone which often polishes well. The thin dark vein through Stone 3 adds to its attractiveness.
A “porphyry” has well-formed crystals visible to the naked eye (called “phenocrysts”), set in a very fine grained matrix. Porphyries develop when magma that has been slowly cooling and crystallising within the Earth’s crust is suddenly erupted at the surface, causing the remaining uncrystallised magma to cool rapidly (see “Textures of Igneous Rocks” in University of Auckland Geology). So a porphyry is intermediate between quickly solidifying fine-grained rocks and the slowly solidifying rocks with uniform larger crystals.
Stone 4 is amygdaloidal, sometimes also known as a volcanic “filled bubbles” stone. It could be basalt.
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, many amygdales are round as the ones in Stone 4 are. Sometimes tumble polishing amygdaloidal stones can be a challenge as the amygdales can come loose and fall out, or the material between close amygdales can crumble. There are two tiny areas in Stone 4 where the latter has occurred but generally it has polished very well.
Stone 5 is an unusual one for Gemstone Beach, in my experience. It kind of looks like petrified mud.
The lighter bits look like plant material. Again, the stone has tumble polished quite well. It’s very hard, though one of the light-coloured thread-like areas turned out to be softish and has worn more quickly than the rest of the stone.
Stone 6 is quite thin, though it’s face is four by three centimetres in size. I wonder if it might be a hydrogrossular garnet, as it is very smooth, dense and very fine grained. One of the key ways to identify a hydrogrossular garnet is by its waxy feel when it is in the rough. However, tumbling removes that waxy feel so I can’t check it.
The next stone (Stone 7) is a classic translucent hydrogrossular garnet but there are also a variety of opaque ones, mixed with other minerals, on Gemstone Beach. 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. Gemstone Beach is well known for its hydrogrossular garnets, and often they are referred to as the beach’s “gems”. In the TumbleStone Two page on this type of stone, I have described seven types of hydrogrossular garnets that I had found up until that date (June 2022). There are links right at the end of that page to two pages of photos of the types. Stone 6 doesn’t easily fit into any of those types but I wouldn’t rule it out as a hydrogrossular garnet.
Stone 7 is a very good example of a semi-translucent hydrogrossular garnet (what I call Type 2). Holding it up to the sun, some light comes through it.
Stone 8 is an example of my Type 4 hydrogrossular garnet. It is opaque, the sun won’t shine through it.
Stone 9 is another opaque hydrogrossular garnet, with an interesting spot on one side:
In the close-up image, the “spot” looks like it’s surrounded by a ring of white crushed quartz that breaks through to a rough surface. However, all of the stone is very smooth, including that area.
Stone 10 is another Type 4 hydrogrossular garnet but with more opaque brown mineral in it than Stones 8 and 9.
Stone 11 is what I call the “porcelain” type of hydrogrossular garnet. It is an opaque stone that looks and feels like porcelain china, often having fine blue-green veins. There’s a line of broken material down the middle of the stone – it can barely be felt and the stone has polished well.
I initially did not think this type of stone (Stone 11) was hydrogrossular garnet. I have checked with rockhound groups online, and a number of people have identified it as including hydrogrossular garnet.
Finally, I am unsure whether Stone 12 is another hydrogrossular garnet or a quartz with mineral inclusions. It is not uncommon for me to be doubtful about identification. However, it is the nature of the thin veins within Stone 12 that make it more likely to be quartz in my mind.
The next Post features another 12 of the Gemstone Beach stones I polished for Michelle.




































































