Rainbow moonstone might be called a "moonstone," but it's actually a transparent form of labradorite. Labradorite and moonstone are both feldspars with similar chemical makeups and crystal structures, but which react to light differently. It can be distinguished from true moonstone by its greater transparency and play of colour, which are qualities of labradorite. This captivating effect is created by layers of internally repeating feldspar that scatter light, producing a mystical blue glow that resembles moonbeams. While high-quality rainbow moonstone is highly transparent, the roughs and tumbles commonly sold are usually less so, and the stone colour of rainbow moonstone is typically a somewhat translucent white or grey. It is also sometimes called "Labradorite Moonstone."
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