Hold a moonstone still and it is nearly colourless. Tilt it and a blue sheet slides across the surface, as if it were lit from inside. Nothing about that blue is pigment.
Moonstone is a feldspar. As the crystal cooled it separated into alternating microscopic layers of two feldspars with slightly different refractive indices. Light entering the stone scatters off those layers and comes back out biased toward blue. The effect is called adularescence. It is structural, not chemical — which is why it moves when you move, and why it cannot be dyed, faked with a coating, or improved by treatment.
Where the blue comes from
Feldspar makes up more of the Earth's crust than any other mineral group. Moonstone is a specific accident within it: an intergrowth of orthoclase and albite that formed as the crystal cooled slowly enough for the two to unmix into layers. Those layers are on the order of a few hundred nanometres thick — comparable to the wavelength of visible light itself.
Light hitting layers at that scale does not simply reflect. It scatters, and shorter wavelengths scatter more strongly. The blue you see is the same physics that makes the sky blue, happening inside a stone the size of a fingernail.
Thinner layers give the pure blue prized in fine moonstone. Thicker layers give the milky white sheen most people picture. Both are the same mineral, cooled at different rates.
Adularia, and a Swiss mountain
The word adularescence comes from Adular, an old name for the Adula massif in the Swiss Alps, where a clear variety of orthoclase feldspar was quarried. Mineralogists named the variety adularia after the mountain, and the optical effect after the variety. So the name of moonstone's defining property is, at two removes, a place in Switzerland.
The decade moonstone owned
Moonstone had been used continuously in Roman, Indian and Ottoman work, but its high point in European jewellery is narrow and datable: roughly 1890 to 1910.
- 1890sArt Nouveau turns away from the faceted, high-contrast diamond aesthetic toward stones that glow rather than sparkle.
- c. 1900René Lalique and Louis Comfort Tiffany both use moonstone heavily — it suits the movement's obsession with translucency and organic line.
- 1910sThe Arts and Crafts workshops in Britain, working in silver rather than platinum, adopt it as a signature stone.
- 1912The American National Retail Jewelers Association standardises the modern birthstone list. Moonstone is given June, alongside pearl.
Both Art Nouveau and Arts and Crafts were, in part, arguments against industrial jewellery. Moonstone fitted because it cannot be mass-standardised: each cabochon has to be oriented by hand so the sheen sits square in the dome. A machine cannot find it.
Rainbow moonstone is not moonstone
The stone sold as rainbow moonstone is white labradorite: a different feldspar, showing labradorescence rather than true adularescence, in flashes of blue, gold and green rather than one moving sheet of blue. It is a trade name, and a durable one. We use it because it is what everyone searches for, and we tell you what it is at the same time.
Looking after it
Moonstone sits at 6 to 6.5 on the Mohs scale — softer than quartz, which means ordinary household dust can scratch it. It also has distinct cleavage in two directions, so a sharp knock can split it along a plane rather than chip it.
Practically: no ultrasonic cleaner, no steam, no chlorinated water. Warm water, a drop of unscented soap, a soft cloth. Store it where it is not rubbing against harder stones.
Frequently asked
What causes the blue in a moonstone?
Adularescence — light scattering off alternating microscopic layers of two feldspars with different refractive indices. It is an optical structure, not a pigment, which is why the blue moves as the stone moves.
Is moonstone hard enough for a ring?
It is wearable but not hard-wearing. At 6–6.5 on the Mohs scale, with cleavage in two directions, moonstone is better suited to rings worn with some awareness than to a ring never taken off.
Is rainbow moonstone real moonstone?
It is a real gemstone but a different mineral: white labradorite, not orthoclase. The name is a trade name and is used industry-wide.
What month is moonstone?
June, in the modern list — shared with pearl and alexandrite.
Can moonstone be treated or dyed?
The sheen cannot be added by treatment, because it comes from internal structure. Some low-grade material is backed or coated to imitate the effect; a genuine sheen moves continuously as the stone is tilted rather than sitting flat on the surface.
Moonstone at Asi8
Sources and further reading: Max Bauer, Precious Stones (1904); Vivienne Becker, Art Nouveau Jewelry (1985); Charlotte Gere and Judy Rudoe, Jewellery in the Age of Queen Victoria (British Museum, 2010). Historical, cultural and mineralogical background only. Nothing here is a medical or therapeutic claim.