Moonstone: The Complete Guide

Moonstone has enchanted humans across cultures for thousands of years. The Romans believed it was formed from frozen moonlight. In Hindu tradition, it is a sacred stone formed from moonbeams. In the Western world, it was among the most fashionable gems of the Art Nouveau period, beloved by Rene Lalique and Louis Comfort Tiffany for its otherworldly glow. Today it enjoys a major renaissance in bohemian, artisan, and fine jewellery alike. For jewellery professionals, moonstone’s romantic appeal and relatively accessible pricing create excellent opportunities across a wide range of clients.

The Science of Moonstone’s Glow

Moonstone is a variety of orthoclase feldspar (a potassium aluminium silicate) belonging to the monoclinic crystal system. Its characteristic optical phenomenon — adularescence — is caused by light scattering from alternating sub-microscopic layers of different feldspar compositions (typically orthoclase interlayered with albite) that form as the crystal slowly cools from its original magmatic temperature.

As these interlayers form, they create a structure of thin parallel lamellae (layers) whose thickness is comparable to the wavelength of visible light. When light strikes these layers, it scatters at specific wavelengths according to the layer thickness. Thin layers (around 100 nm) produce blue adularescence; thicker layers produce white. The scattering creates the impression of a floating glow beneath the surface — as if the light source is inside the stone rather than outside it.

The finest adularescence appears as a deep blue glow over a colourless or near-colourless body, seen in the Ceylon (Sri Lankan) moonstones that have been prized since antiquity. As layer thickness increases and composition becomes less uniform, the glow shifts from blue toward white, producing the more common white moonstone with a milky sheen rather than a true floating blue.

Major Varieties and Sources

Ceylon Moonstone (Sri Lanka)

Sri Lanka produces the finest moonstone — colourless to near-colourless body with deep blue adularescence. These stones represent the ideal: the body is transparent rather than milky, providing maximum contrast for the blue glow. Fine Ceylon moonstones of 5-10 carats with strong blue adularescence and good transparency are increasingly rare and command premium prices. Sri Lanka also produces grey, peach, and white moonstone in addition to the classic blue-adularescent variety.

Indian Moonstone

India produces large volumes of commercial moonstone in a range of body colours including grey, peach, orange, and green. Indian moonstones tend to have milkier body colour and white to pale adularescence rather than the blue glow of fine Ceylon material. Rainbow moonstone from India (technically labradorite rather than orthoclase) shows multicoloured labradorescence over a white to near-colourless body and has its own distinct appeal.

Other Sources

Myanmar (Burma), Madagascar, Tanzania, Brazil, and the United States (Virginia, North Carolina) also produce moonstone. Myanmar produces some fine blue-adularescent material. Madagascar has emerged as a significant source of various quality levels. Australian moonstone (sanidine feldspar) occupies a niche collector category.

Quality Assessment

Moonstone quality is assessed primarily on adularescence quality (intensity, colour of glow, centring) and body colour transparency. The strongest, most centred, most deeply blue adularescence over the most transparent, clean body represents the ideal. The adularescence should be evaluated from directly above the stone and at various angles — it should sweep and move as the stone is tilted, not appear as a static white patch.

Inclusions in moonstone are common: centipede inclusions (short, fine tube-like inclusions in a stacked arrangement resembling a centipede), stress fractures, and other features. Some inclusions are acceptable; centipede inclusions in small quantities are diagnostic of natural moonstone. Heavily included or fractured material reduces commercial grade significantly.

Rainbow Moonstone: A Separate Category

Rainbow moonstone is a variety of labradorite (plagioclase feldspar) that shows multicoloured labradorescence. While sold commercially as “moonstone” (and widely accepted under this name), it is technically a different mineral species from orthoclase moonstone. The distinction matters for sophisticated buyers but is not commercially significant in most retail contexts. Rainbow moonstone shows more dramatic and varied colour play than most orthoclase moonstone but lacks the deep blue adularescence of fine Ceylon material.

Durability

Moonstone has a Mohs hardness of 6-6.5 and two directions of cleavage (related to the feldspar crystal structure). It is a somewhat delicate stone for jewellery use, suitable for earrings and pendants without concern but requiring care in ring settings. Moonstone should not be cleaned in ultrasonic cleaners. It is sensitive to pressure along cleavage directions and can split if a ring is bent or struck against a hard surface.