Slope Point is known for its rhyolites. Some appear to be rather plain-looking, some are spherulitic, and some I think are eye-catching stones, often being colourful stones with interesting lines and patterns. Stone #12 was found on a Slope Point beach in 2021 and completed the polishing process early this month. It is 4 cms long on its longest axis. There are some tiny patches that are unpolished or not completely smooth but I didn’t want to make it too small with further tumbling. I think this stone is a rhyolite, but I am unsure:
Rhyolite is a volcanic rock that has a wide range of different colours, textures and structures. Certainly this stone has some complexity and is of volcanic origin.
Rhyolite is the extrusive equivalent of granite. It is formed from magma rich in silica that is extruded from a volcanic vent to cool quickly on the surface rather than slowly in the subsurface. It is the most silica-rich of the volcanic rocks. Generally fine-grained in texture, rhyolite may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass.
Rhyolite is a particularly versatile mineral, also appearing in the form of obsidian, ignimbrite and pumice, depending on the circumstance of its geological history.
Why do I hesitate to identify this as a rhyolite stone? Most of the literature on rhyolite does not refer to it a being colourful. It is usually labelled as “light-coloured” and “pale”, and even photos of flow-banding appear rather plain (see page 64 of John Farndon, 2015, “The Illustrated Guide to Rocks & Minerals” below):
The Slope Point rhyolite stones I saw in Jack Geerling’s collection (Winton, Southland) did not include any especially colourful ones, and the New Zealand literature is similar:
However, every now and then I see a colourful stone posted in Facebook Groups that looks similar to Stone #12 and it is identified as rhyolite. So I’m not discounting it as rhyolite yet.
In general, there is a great diversity of stones within each basic type, especially when dealing with individual stones found on a beach. In a sense, every one is unique. There are also often small bits of a number of types of rock material mixed in a stone, and clear identification cannot always be made.
Stone of the Day #13 is here. The Index to the January 2022 Stone of the Day Series is here.
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