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Moonstones are colourless, milky white or sometimes grey or peach coloured. The prized blue - adularescence or schiller - comes from deep within the stone when viewed at certain angles. The blue is not an intrinsic mineral colour. It is instead a form of iridescence arising from multi-layer interference of light.
Moonstone is a feldspar, within the stone there are domains of alternating layers of slightly different composition.
Feldspars are complex sodium potassium aluminium silicates. Albite (sodium rich) and orthoclase (potassium rich) are compositional extremes or end members of feldspars. As a feldspar melt cools, albite solidifies first leaving an adjacent potassium rich liquid which then forms an orthoclase tending solid. That leaves an adjacent albite rich liquid. The outcome can be a regular stack of alternating layers each less than a micron thick of sodium rich and potassium rich material.
The alternating layers have slightly different optical properties and light passing through them is reflected and refracted at their interfaces. The reflected light waves interfere to produce the adularescence colours.