This is a slightly expanded version of my letter “V” 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 “V is for The Chevron Shape of Trace Fossils”.
V is for Veined Vein?
This is a Gemstone Beach stone I photographed in April 2023 when I was collecting images for use in the Series “Gemstone Beach and its Stones: An Introduction for the Passing Motorist” – it is Stone W92 in this Post. The stone has an untidy white vein (of quartz or calcite maybe?) running through the middle. And it looks like this vein itself has a thin brighter-white vein running through it. However, maybe the thin vein is a denser or more intense thread of the same mineral in the wider vein. Or maybe some tiny bubbles of water and/or air are present in that thin line, making it brighter. It is often hard to know what is going on in the stones we find.
Veins are a common yet fascinating feature in stones. This can be illustrated by three other Gemstone Beach finds that feature in the “Gemstone Beach and its Stones: An Introduction for the Passing Motorist” Series (see Stones W87, W103, W110 in this Post). In the first below, there are veins of different colours, thicknesses and direction. In the second are irregular opaque and transparent quartz veins. The third has cross-cutting veins:
Veins are infilled cracks. When rock fractures due to pressure and stress, cracks of different sizes open up. This allows hydrothermal fluids, carrying dissolved minerals, to enter the gaps in the rock. Over time, the dissolved minerals precipitate out of the hot water. Crystals, such as quartz and calcite, grow within the cracks and fill them. Often, as the stresses on the rock change their orientation over time, veins will crosscut and even overlie each other, though veined veins are probably very rare. For a reasonably technical account of vein formation see Alex Strekeisen, with Wikipedia having a more easily read account. Note that, in contrast to a vein, a “band” is a layer that is an integral part of the original stone, such as a layer of differently coloured sediment in a sedimentary stone.
The next Post in this Series is “W is for Wacke”. An Index for this Alphabetical Series is here.
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