Polishing Some Gemstone Beach Stones for Michelle: Stones 13 to 24

Here are another 12 of the 51 stones that I recently polished for Michelle (numbered 13 to 24 in the photo below, right). A previous Post featured 12 of her stones as well. Michelle collected these from Gemstone Beach in Southland. She lives near this beach and often visits it.

Stone 13 is just under three centimetres long and is very smooth. It is what I call a “two-sider”. Each side is very different. This contrasts with a “double-sider” where both sides of a stone are very similar.

One side of Stone 13 is a deep brown. The other side is mainly whitish, though at the edges you can see the brown mineral from the first side. The first (brown) side also displays a complex structure, as if part of the stone’s material has melted and flowed over the rest. A careful examination of the second (whitish) side reveals hints of the same structure there. The stone may be quartz with brown iron oxide staining.

Stone 14 is a small brecciated jasper with some quartz in it. As Learning Geology notes, jasper is an opaque form of chalcedony, a cryptocrystalline form of silica. Chalcedony itself is usually described as semi-transparent or translucent. The incorporation of minerals, such as dark red iron oxides, provides jasper with its opaque nature.

Stone 14 has a light brick-red hue, appearing to lack the dark grey metallic hematite that would give it a deeper red colour. It is the second smallest of the 51 stones, two centimetres long, and polished very well except for one tiny rough area.

Stone 15 is a bit bigger, and tumble-polished very well, being completely smooth and shiny. It has a gorgeous character. There are some thin delicate veins of red and yellow minerals in it, along with black specks and a hint of green.

Another light coloured pebble, Stone 16 is bigger still, at four and a half centimetres long. The lines within it make it an interesting stone.

There are tiny quartz crystals in the stone – they sparkle in the light.

Stone 17 is the most colourful stone of the whole 51. Half of it is intense purple. It is also brecciated, with small fragments of white quartz within it. The purple could be manganese or iron oxide or something else, but I don’t know enough to say much more.

It has tumbled well, ending up nice and shiny. However, the second side of the stone (above) has a gash in it, at the bottom left, and there is also a small rough area along the bottom. I could have eventually removed these “imperfections” by tumbling the stone longer in coarse grit but then the stone would have ended up a lot smaller.

Stones 18 and 19 also have some of that deep intense purple in them. They are both basically quartz which shows through as white. Stone 18 has some yellow flashes in it as well, with the smaller Stone 19 having a hint of green along the top, maybe from the mineral epidote.

Stones 20 and 21 are also purple, with that colour infused throughout them. But they are quite different stones otherwise. Stone 20 is a metamorphic stone of some kind.

Metamorphic rocks are one of the three main rock types, the other two being sedimentary and igneous. As University of Auckland Geology notes: “Metamorphism is the alteration of pre-existing rocks in the solid state due to changes in temperature and pressure. Under increasing temperature and/or pressure existing minerals become unstable and break down to form new minerals.”

In contrast, Stone 21 is igneous, probably volcanic.

Igneous rocks form from the cooling and solidification of magma or lava. There are two kinds of igneous rocks. The first is “volcanic”, having come from volcanoes and therefore solidified relatively quickly (also called “extrusive”). The second is “plutonic”, formed underground from molten magma and therefore solidified relatively slowly (also called “intrusive”). Stone 21 is most likely to be rhyolite, a reasonably common volcanic rock. Rhyolite is high in silica and so often polishes well – the glass-like obsidian is one form of rhyolite (as is the light pumice). However, small crystals within rhyolite can wear away quickly during tumble-polishing and leave tiny holes in the end product. There are a variety of types of rhyolite stones on Gemstone Beach. This one, Stone 21, has polished well. Geology.com provides good information on rhyolite in general. More information on types of rocks is given in the TumbleStone Two Post “Brief Introduction to Geological Terms for Stones” in the Series “Gemstone Beach and its Stones: An Introduction for the Passing Motorist”.

I suspect that Stone 22 will be the one that Michelle likes the most of the 51 in this batch. She seems to especially love the pink stones from Gemstone Beach. I gifted her the one in this Post after I met her on the beach as I was carrying it with me along with some other polished Gemstone Beach specimens. It and Stone 22 are thulite.

Thulite is the national gemstone of Norway where it was first discovered in 1820. It is the pink to reddish variety of the mineral zoisite. As  Minerals.net  reports, “The color of thulite is caused by the element manganese in its composition. Thulite often occurs associated with quartz, and is sometimes mottled with streaks or spots of white quartz.” You can read more about it on Wikipedia and Gem Adventurer. There are sometimes clouds of intense bright pink in Gemstone Beach thulites. Others I have found are much lighter in colour. In my experience, many of the brighter stones don’t tumble-polish well, coming out rough and non-shiny, while the lighter coloured ones often come out much better.  Fortunately, Stone 22 has turned out smooth and shiny.

The final two stones considered in this Post are examples of the greens that can be found on Gemstone Beach. Often it is the mineral epidote that causes the green hues. Stone 23 is a nice smooth stone, a circular shape with a diameter of about two and a half centimetres. There are a number of other minerals apparent in it.

Stone 24 has large areas of white quartz, some intense green patches and some threads of light and dark green. It is very smooth as a result of the tumble-polishing process.

There is another batch of Michelle’s stones about a week away from being tumble-polished – see here for a Post on these. And there is one more batch that needs careful sorting before going to the final polish stage – some of these may not prove suitable and will be sent back to Michelle unpolished. I will post photos of selected specimens once polishing is done.