Rainbow sheen is the multicolour iridescent optical effect observed in transparent to translucent white labradorite, the plagioclase feldspar widely marketed in the trade as rainbow moonstone. The sheen is technically a form of labradorescence — the same phenomenon that produces the colour flashes of grey labradorite from Labrador and Finland — but the host material here is the sodium-rich, transparent end of the labradorite series rather than the more familiar grey calcium-rich material. The result is a striking optical effect in which the pale, almost colourless body of the gem carries flashes of vivid blue, violet, green, and occasionally gold visible at specific angles.

Optical mechanism

The rainbow sheen arises from light interference at the boundaries of submicroscopic lamellar twinning planes within the plagioclase feldspar. Plagioclase undergoes a complex phase transition during cooling: an originally homogeneous high-temperature crystal unmixes into alternating lamellae of slightly different composition (more sodium-rich and more calcium-rich), with the twinning planes oriented in a specific crystallographic direction. When light enters the gem and reflects from successive twinning planes, the reflections interfere; constructive interference at specific wavelengths produces the colour flashes.

The same mechanism produces the better-known dark labradorescence of grey Finnish labradorite (sometimes traded as spectrolite). The difference between rainbow sheen and the grey-host labradorescence is principally the body colour of the host material — transparent white in the rainbow moonstone case, opaque grey in the Labrador case — and not the underlying optical mechanism.

Distinction from true adularescence

Rainbow sheen is sometimes confused with the adularescence of true moonstone (alkali feldspar), but the two are visually and mechanically distinct. True adularescence is a soft, billowy white-to-blue glow that floats over the cabochon as it is rotated, produced by light scattering at the boundaries of submicroscopic alkali-feldspar lamellae. Rainbow sheen, by contrast, is sharper, more directional, and more vividly coloured, with flashes that appear and disappear over short rotation arcs.

Both effects are structural rather than pigmentary; both depend on the orientation of the cabochon relative to the internal lamellar structure. But the spectral character is distinguishable to the trained eye: adularescence is dominantly white with a soft blue cast, while rainbow sheen is dominated by saturated spectral colours including violet, green, and gold in addition to blue.

Cutting for sheen

The cutter orients the rough so that the lamellar twinning planes lie roughly parallel to the base of the cabochon. The dome of the cabochon then maximises the surface from which the sheen displays as the gem is rotated. Cutting at the wrong orientation produces a transparent or translucent stone with little or no rainbow effect, and skilled cutters work the rough specifically to bring out the labradorescence.

The phenomenon is angle-dependent and best displayed under direct point-source illumination rather than diffuse light. Under cloudy daylight or diffuse indoor lighting the sheen may be subdued; under direct sunlight or a focused jeweller's light it shows vividly.

Sources and trade

The principal sources of rainbow-sheen labradorite are India and Madagascar. Indian material — particularly from southern Karnataka and Tamil Nadu — supplies the bulk of the commercial trade; Madagascan material is encountered in higher-quality grades. The trade designation rainbow moonstone is now firmly established for this material despite its mineralogical inaccuracy, and the sheen is the principal quality factor distinguishing fine commercial pieces from ordinary translucent labradorite without significant iridescence.

Quality factors

The principal quality factors for rainbow-sheen labradorite are the intensity and colour range of the iridescence, the transparency of the host material, and the absence of obvious inclusions or fractures visible without magnification. Top material shows electric blue or violet sheen on a transparent colourless body and a wide range of viewing angles over which the sheen displays vividly; commercial-grade material shows weaker sheen, often only blue or only gold rather than the full spectral range, and a more translucent rather than transparent body.

Pricing varies widely: top-grade rainbow-sheen labradorite cabochons of significant size (10 carats or more) with electric blue-violet sheen on transparent body command meaningful per-carat prices in the artisanal jewellery market, while ordinary commercial material is available at modest prices through Indian and Sri Lankan dealers.

Care and setting

Rainbow-sheen labradorite has hardness of 6 to 6.5 on the Mohs scale and perfect cleavage in two directions at near-right angles, like all feldspars. The combination of moderate hardness and perfect cleavage requires careful setting and protection from impact and thermal shock. Bezel settings are generally preferred to prong settings, and ultrasonic and steam cleaning are not recommended. Mild soap, warm water, and a soft brush are appropriate for cleaning.

Identification

Rainbow-sheen labradorite is identified through standard gemmological testing: refractive index in the range 1.555 to 1.572, specific gravity around 2.68 to 2.72, and characteristic biaxial optical character. The material is non-fluorescent under most ultraviolet conditions, though some specimens show weak orange or red fluorescence under shortwave UV. The combination of these properties with the visual labradorescence is diagnostic.

Distinguishing rainbow-sheen labradorite from true alkali-feldspar moonstone in cut material is straightforward by refractive index measurement (alkali-feldspar moonstone has slightly lower RI, around 1.518 to 1.526). The optical character of the sheen — the saturated multi-spectral colours of labradorescence versus the soft white-blue of adularescence — is also diagnostic to a trained eye even without instrumental measurement.

Further reading

• GIA — feldspar reference
• GIA Gems & Gemology — labradorite and moonstone articles
• International Gem Society — labradorescence