Moonstone — Adularescent Feldspar's Floating Blue Light
Moonstone — Adularescent Feldspar's Floating Blue Light
An optical phenomenon, a June birthstone, and the original literary jewel
Moonstone is the trade name for an alkali feldspar — usually a potassium-sodium intergrowth in the orthoclase-albite series — that exhibits adularescence: the soft, billowy, blue-to-white sheen that appears to float across the surface of a properly cut cabochon and shifts as the stone is moved through the light. The phenomenon is the optical signature of the variety, the feature that has carried moonstone from antiquity through every revival of romantic, Aesthetic, and Art Nouveau jewellery design, and the principal driver of price in the modern trade. Without adularescence, the same feldspar is just feldspar.
Mineralogy
Moonstone is most commonly orthoclase, KAlSi3O8, but the gem-grade material is essentially a fine intergrowth of K-rich orthoclase and Na-rich albite produced by exsolution as the originally homogeneous feldspar cooled through subsolvus temperatures. The lamellar K-Na intergrowth, with lamellae on the order of hundreds of nanometres thick, scatters incident light at the lamellar boundaries; where the spacing is in the right range relative to visible wavelengths, the scatter favours shorter wavelengths and produces the characteristic blue floating sheen.
The species is monoclinic with hardness 6 to 6.5 on the Mohs scale, specific gravity around 2.55 to 2.63, and refractive indices of approximately 1.518 to 1.526. Cleavage is good in two directions at near-right angles. Some moonstone is technically oligoclase or peristerite (intermediate plagioclase compositions); the optical principles are similar across these closely related feldspars.
Adularescence and its quality
The desirability of moonstone is governed almost entirely by the quality of its adularescence. The finest stones show a clean, electric-blue sheen that floats on a near-colourless transparent body, visible from a wide range of viewing angles, with the sheen centred over the dome of the cabochon when the stone is properly oriented. Lesser material shows a silvery white sheen on a more translucent, milky body, with the effect washing out at oblique angles.
Cutters orient the rough so that the lamellar planes lie parallel to the base of the cabochon, with the dome curvature optimised to capture the sheen. Faceted moonstone is rarer in commercial work but possible with very clean, well-oriented rough; the play of the sheen through the facets produces a different optical character.
Sources
The classical source for fine blue-sheen moonstone is Sri Lanka, particularly the Meetiyagoda area in the south of the island, which has supplied the international trade since at least the nineteenth century. India produces large quantities of commercial moonstone, including white moonstone and the orthoclase-oligoclase intergrowth marketed as rainbow moonstone (which is technically labradorite — see the separate entry). Madagascar, Tanzania, Myanmar, and the United States produce additional commercial material. Adularia from alpine clefts in Switzerland and Austria is mineralogically related but rarely cut as a gem.
Trade categories
The trade distinguishes broadly between blue-sheen moonstone (premium), white-sheen moonstone (commercial), and rainbow moonstone (a labradorite displaying multi-coloured iridescence rather than true adularescence). Within blue-sheen material, transparency and the saturation and centring of the sheen drive value. Star moonstone — material showing a four-rayed or six-rayed asterism in addition to adularescence — is rare and commands premiums.
Care and wear
At hardness 6 to 6.5, moonstone is among the softer commonly-set gems and requires care. We recommend setting fine moonstone in pendants and earrings as a first preference, and in protected ring designs (bezel rather than prong) where ring use is the goal. Cleaning should be by mild soap and warm water with a soft brush; ultrasonic and steam cleaning are not recommended because of cleavage and the risk of thermal shock. Avoid exposure to harsh chemicals.
In the trade
Moonstone is one of the official birthstones for June (alongside pearl and alexandrite), a position cemented by the American National Association of Jewelers' 1912 list and reaffirmed by the Jewelers of America. The gem was a defining feature of Art Nouveau jewellery — Lalique's moonstone work is the touchstone — and continues to feature in design-jewellery and contemporary collections that draw on those references. Premium Sri Lankan blue-sheen material is in genuinely short supply and prices have risen substantially in recent decades. White and commercial moonstone remains widely available at accessible prices.