Moonstone and the trick of adularescence

Moonstone is a variety of orthoclase or, less commonly, albite feldspar, formed from microscopic alternating layers of two feldspar types within the crystal. Light entering the stone scatters off these layers and produces adularescence, the glow that seems to hover under the surface and move as the stone is tilted.

At roughly 6 to 6.5 on the Mohs scale it sits well below quartz and considerably below sapphire, and feldspar also has cleavage planes, meaning a sharp knock can crack the stone along a preferred direction rather than simply scratching the surface.

Classic moonstone has a colorless to milky white body with a blue adularescent glow, the most prized combination in the trade. Body color can also run to grey, peach, champagne or a pale green, and the strength and color of the glow, rather than the body color alone, is what separates a fine stone from an ordinary one.

The blue sheen is strongest when the stone is cut as a smooth, high-domed cabochon oriented correctly relative to the internal layering, which is a cutter's skill rather than a treatment.

Moonstone often contains fine internal stress fractures, sometimes called centipedes for their branching appearance, which are typical of the material and not usually treated as a serious flaw unless they reach the surface.

Moonstone is essentially untreated. Its defining quality is structural rather than color-based, so there is no equivalent of heat treatment or irradiation applied to it in normal trade practice, and any dyeing to alter body color should be disclosed if suspected.

Because it is relatively soft and has cleavage, moonstone is better suited to earrings, pendants and occasional-wear rings than to a ring worn daily doing housework or manual tasks. Internal stress fractures can also grow over time with rough handling.

We advise against ultrasonic and steam cleaning for moonstone. A soft cloth or a gentle rinse in lukewarm water is enough, and moonstone jewelry is best stored separately from harder stones that could scratch its relatively soft surface.

Moonstone is most strongly associated with Art Nouveau, from roughly the 1890s to around 1910, when its soft, mysterious glow suited the movement's fascination with nature, moonlight and the female form, often set in silver-topped gold or platinum alongside enamel and pearls.

It also appears in Arts and Crafts jewelry of the same general period, usually in simpler hand-wrought silver settings, and saw further revivals in the 1960s and 70s as part of a broader interest in unusual, non-traditional stones.

Synthetic spinel and various glass imitations have been sold as moonstone, along with a milky chalcedony sometimes marketed as a cheaper substitute. None of these show true adularescence, the specific rolling blue glow caused by feldspar's internal structure, though a good imitation can look convincing at a glance.

Rainbow moonstone, a trade name for a variety of labradorite feldspar with a similar sheen, is a related but distinct material and worth naming correctly, since true moonstone and rainbow moonstone are not identical in composition even though the visual effect is closely related.