Rainbow moonstone tumblestone
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Moonstone is a sodium rich plagioclase feldspar. It demonstrates a flash of colour when moved under the light. This is called labradorescence. The optical effect is due to exsolution. Plagioclase is part of a solid solution series -- it has different proportions of calcium and sodium within its structure. Sodium and calcium ions have different ionic charges, but a similar ionic radius. At high temperatures the mineral is more concerned with the size of the radius. However, once it has cooled down to make a solid, the differences in charge make the material energetically unstable. In order to form a more stable structure, the plagioclase separates out into a more stable structure. This creates small gaps which have the same wavelength as blue light.
Average weight, 7g. All prices are per item. Photographs are representative of the items on offer, but the item you buy may not be the one in the photograph