Labradorescence is the optical phenomenon of broad, schiller-like iridescent colour displayed by some specimens of labradorite, the calcium-rich variety of plagioclase feldspar. The phenomenon is the diagnostic feature that gave labradorite its commercial value and is one of the four named optical phenomena recognised in the gemmological literature alongside aventurescence, opalescence and adularescence.

The mechanism

Labradorescence is a thin-film interference phenomenon caused by the lamellar structure of the feldspar. Labradorite is a member of the plagioclase solid-solution series, with composition between 50 and 70 percent anorthite (the calcium endmember). At slow cooling, this composition exsolves into thin alternating lamellae of two slightly different feldspar phases, each tens to hundreds of nanometres thick. Light entering the stone reflects from the boundaries between the lamellae, and the interference between the reflected wavefronts produces the colour we see. Different lamellar thicknesses produce different colours: thicker lamellae shift the displayed colour toward longer wavelengths, thinner toward shorter. Specimens with a range of lamellar thicknesses display a corresponding range of colours.

The phenomenon was first explained mechanistically in the early twentieth century, with the precise structural studies coming from x-ray and electron-microscopy work in the 1950s and 1960s. The Bowen-Tuttle phase diagram for the plagioclase series and subsequent transmission electron microscopy of labradorescent material established the lamellar thickness range and the relationship to displayed colour.

Trade colour ranges

The colour range observable in labradorescent labradorite includes blue, green, gold, orange, red, violet and, in exceptional specimens, a near-complete spectrum on a single stone. The most common displays are blue and gold; reddish and violet displays are less common and command commercial premium. The Finnish material trade-named spectrolite, mined principally at Ylamaa in southeastern Finland, displays the broadest range of colours and is the trade reference for top-quality labradorescence.

The base colour of the host stone is also relevant: dark grey to black hosts produce more dramatic display than pale grey hosts of equivalent lamellar structure, since the dark base absorbs background light and lets the iridescent colour stand out. Cutting and orientation matter: the schiller-bearing plane within the stone must be aligned roughly parallel to the cabochon's flat base for the display to be visible from the top.

Distinguishing from related phenomena

Labradorescence is distinct from aventurescence, the metallic glittering effect produced by inclusions of hematite, goethite or copper in feldspar. It is also distinct from adularescence, the floating soft glow seen in moonstone, which arises from a similar but finer-scale lamellar structure in orthoclase-albite feldspar. Schiller is a general term sometimes applied to all such iridescent feldspar effects, and is sometimes used interchangeably with labradorescence; in the strict gemmological literature labradorescence is reserved for the broad iridescent colour seen in labradorite specifically.

Treatment and synthesis

Labradorescence is not enhancable by any conventional treatment; the phenomenon is structural and depends on the lamellar exsolution that took place during the original cooling of the host rock. There is no synthetic labradorite of commercial significance.