Moonstone

Moonstone is the gem variety of feldspar celebrated for adularescence — a soft, billowy sheen of light that seems to float beneath the surface and move as the stone is turned. The effect arises from light scattering off microscopic alternating layers of two feldspars (typically orthoclase and albite) that unmixed as the crystal cooled.

Properties

Moonstone is most often a potassium feldspar close to orthoclase; it is monoclinic, moderately hard (6–6.5) and has two good cleavages. The most prized stones are near-colourless and transparent with a vivid blue sheen, though 'rainbow moonstone' (a labradorite) shows multicoloured flashes.

Occurrence

Sri Lanka and India are the historic sources of fine blue-sheen moonstone; Myanmar, Madagascar and Tanzania also produce gem material.