This is an expanded version of my letter “S” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “S is for Slope Point Stone”.
S is for Scoria
On my current morning walks on Kai Iwi Beach (near Whanganui) with dog Jasper, I am getting a more detailed understanding of the range of stones to be found here. They are quite widely scattered but that makes it easier to spot interesting ones. For more on this beach and its stones, see “The Iron-Stained Stones of Kai-Iwi Beach” and “K is for Kai Iwi Beach Stone”.
About three weeks ago I became intrigued with a type of stone I had previously just ignored, and I think it’s scoria. It’s like pumice but harder, more dense, and feels rougher. Most of these stones are dark red, some are grey, and they often have small black and white crystals in them. Despite their roughness and many tiny holes, I decided to collect a few and tumble polish them, just to see the results. Some are now undergoing a 220 grit tumble (see NOTE ON TUMBLING below).
As Wikipedia informs me, scoria is a pyroclastic, highly vesicular, dark-coloured volcanic rock formed by ejection from a volcano as a molten blob and cooled in the air. It is typically dark in colour (brown, black or purplish-red), and often basaltic in composition. Scoria has a relatively low density as it is riddled with vesicles (tiny hollows from gas bubbles), but in contrast to pumice, scoria always has a specific gravity greater than one and sinks in water.
Coincidentally, my wife bought some scoria recently for the greenhouse floor – it’s brighter and more chunky than the water-smoothed specimens I find on Kai Iwi Beach. Commercial scoria often comes from the Auckland volcanic field which includes hills of this type of rock, and it is sometimes excavated for building materials and landscaping. See Geotrip to “Big King Scoria Cone”.
Another coincidence: I live on Rapanui Road, and on Rapa Nui/Easter Island, reddish scoria was used to carve the pukao (topknots) for the famous moai statues, and even for the main bodies of some moai – see “Constructing the Rapa Nui moai”.
An interesting comment on my Post in the Facebook Group was made by Bruce Taylor, a local fossil hunter: “Some of the stones found here used to be moa, as in they are from fossilised bones. Rare and hard to spot. Exceptional shark teeth also with fossilised root that is now stone. Wood, seal, dolphin, penguin, all rare but found and in stone. Examples can be seen at the camp ground.”
NOTE ON TUMBLING: I tumbled some scoria stones for a few days in 220 silicon carbide grit. The resultant slurry contained a lot of fine fragments that had come off the stones. Many of the tumbled stones felt relatively smooth, certainly a lot smoother than beforehand. Five of the more interesting ones:
Four more:
However, close inspection showed that, as could have been anticipated, there were still a lot of tiny holes and further tumbling would not have resulted in a smooth polished product. Tiny bits of scoria loosened throughout the process would also contaminate later barrels and loads, an issue identified by John Taylor (Scotland) in a comment on my Facebook Group post. I have therefore decided not to proceed any further with them.
The next Post in this Series is “T is for a Type of Tiny Tectonic Breccia?“. An Index for this Alphabetical Series is here.
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