An igneous stone, most likely rhyolite, this stone was found on Gemstone Beach or the coast just north of there along to the Waimeamea River mouth. I initially learned to recognise rhyolite stones at Slope Point and then, afterwards, started to notice them at Gemstone Beach as well. Stone of the Day #20 is a type of rhyolite that I now see each time I fossick at Gemstone Beach (there’s a very similar stone near the end of this Southern Sojourn Post).
It is not always possible to get a good smooth polished rhyolite stone but this one turned out quite good. The small light-coloured crystals have interesting shapes.
Rhyolite is a volcanic “felsic” rock, having a high silica and feldspar content. (“felsic” comes from “fel” from feldspar and “sic” from silica). “Felsic” rocks are not only silica rich but also tend to have a lot of aluminium, sodium and potassium, often giving them a lighter colour. Rhyolite is an unusual mineral as it can take quite different forms – University of Auckland Geology notes that it can be pumice, ignimbrite, obsidian or flow-banded etc., depending on whether the molten rock has erupted or flowed, what its gaseous content is, and how quickly it has cooled. Geologyscience.com notes that rhyolite is the extrusive form of granite, but that rhyolite has a fine-grained texture with phenocrysts, which are small crystals sometimes embedded within the rock (as apparent in Stone #20).
Go to Stone of the Day #21.
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