The first Post in this Series is Stay-at-Home Day One.
Stone Four is a very unusual stone. I have not come across one like it before and it is a bit of a mystery to me.
I found Stone Four on Gemstone Beach on 9 June 2019 – I know it was 9 June because I took a photo of it on that day and the date is listed in the photo’s properties. Its appearance then was quite different:
The stone as found on the beach had a rough surface with light brown patches across it. But under those patches seemed to be a slab of a glass-like texture. In some ways, the smoothness of that underlying texture reminded me a bit of the hydrogrossular garnets which can be found on Gemstone Beach but hydrogrossulars don’t have this kind of surface. In other ways, the stone’s underlying glassiness reminded me of the kind of agate stones found on Birdlings Flat in Canterbury but agate is extremely rare on the south coast and Stone Four feels heavier than agate.
When held up to the light, it is apparent that there is a large black inclusion filling the right-hand side of the stone. In fact, this inclusion comes to the surface at the top of the stone, one-third of the way along from the right in the following photo (it can also be seen as a black patch in the second photo at the top of this Post):
My guess at this stage is that the glassy part of Stone Four consists of a kind of cryptocrystalline quartz, maybe chalcedony. Quartz is the second most abundant mineral in Earth’s continental crust, behind feldspar. It is composed of silicon and oxygen atoms. Cryptocrystalline quartz is made up of crystals too small to be seen, and it often looks like porcelain. Chalcedony is the general name given to this kind of quartz and with the additions of colours and patterns it makes up a wide variety of rocks, including carnelian, agate and jasper. The lack of any pattern and the overall colour of the glassy part of Stone Four leads me to suggest that chalcedony could be the appropriate identification for it. [May 2022: I now believe this is likely to be hydrogrossular garnet, after comments from members of the Facebook Group, New Zealand Lapidary, Rocks, Minerals, Fossils.]
The next Post in this Series is Stay-at-Home Day Five.
A very different looking stone. A rare find .
Indeed. I often am unsure whether to polish a stone like this as I can’t anticipate what will transpire when the outer layer is removed.