Moonstone
Feldspar with a blue-white glow that floats under the surface — and it is two intergrown feldspars scattering light between them.
Moonstone is a feldspar showing adularescence — a soft blue-white light that appears to float below the surface and moves as the stone is turned.
The cause is structural rather than chemical. The stone is an intergrowth of two feldspars, orthoclase and albite, which separated into alternating layers as it cooled slowly. The layers are of a thickness comparable to the wavelength of light, and light scattering between them produces the glow. Thinner layers scatter blue, which is why the finest material is blue rather than white.
So the effect depends entirely on cooling history: a feldspar of identical composition that cooled quickly shows nothing at all.
How to identify it
Adularescence is diagnostic. Feldspar's two cleavage directions at nearly ninety degrees separate it from chalcedony and from glass imitations, and its hardness of around 6 is noticeably lower than quartz.
Always cut as a cabochon, and oriented so the glow appears over the dome — an incorrectly oriented stone shows the effect at the side or not at all.
Uses
Gemstone, in cabochons and beads. Its moderate hardness and easy cleavage make it a stone for earrings and pendants rather than for rings.
Heavily used in Art Nouveau jewellery, and it is one of the materials most associated with that period.
Name origin
For the appearance, in essentially every language. The mineral name adularia, and the term adularescence with it, comes from the Adula massif in Switzerland.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is a variety of
- Orthoclase — mineral · an intergrowth of orthoclase and albite in layers the thickness of a wavelength of light, which is what produces the glow
is commonly confused with
- Rose quartz — mineral variety · both translucent and cabochon-cut; moonstone has two cleavages and a directional glow where rose quartz has neither
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing