Labradorite is the gem variety of plagioclase feldspar, a continuous solid-solution series between albite (sodium endmember, NaAlSi3O8) and anorthite (calcium endmember, CaAl2Si2O8). The labradorite composition lies between approximately fifty and seventy percent anorthite, classifying it as one of the more calcium-rich members of the series. Its principal gemmological interest is its display of labradorescence, a broad iridescent colour caused by thin-film interference in the stone's lamellar microstructure.

Physical and optical properties

Labradorite has hardness 6 to 6.5 on the Mohs scale, refractive index 1.559 to 1.573, birefringence 0.008 to 0.010, specific gravity 2.68 to 2.72, and triclinic crystal system with two well-developed cleavages. The base colour ranges from pale grey through dark grey to nearly black, with a smaller proportion of specimens showing pale blue, brown or yellow body colour. Lustre is vitreous on fresh surfaces and slightly pearly on cleavage planes.

The diagnostic optical feature is labradorescence, discussed in its own entry. The phenomenon depends on the lamellar exsolution structure of the calcium-rich plagioclase composition during slow cooling, and is absent or weak in specimens whose composition lies outside the labradorite range.

Localities

The type locality is the Tabor Island and Nain area of the Labrador coast, where Moravian missionaries first collected the material in the late eighteenth century. The locality remains a significant producer of labradorite, generally with strong blue, blue-green and gold schiller against a dark base.

The Finnish locality at Ylamaa in southeastern Finland produces material trade-named spectrolite, marketed by Lapponia and other Finnish design houses since its mid-twentieth-century discovery. Spectrolite shows particularly broad iridescent colour ranges, often displaying red, violet, green, blue and gold within the same specimen, and commands a commercial premium.

Other significant localities include Madagascar, where labradorite from the southern part of the country is mined in commercial volumes for both lapidary and architectural use; Russia and Ukraine, where Volyn pegmatite-related labradorite has been extracted for centuries; the Adirondacks of New York and the Larvik anorthosite of Norway. Australia, India and Mexico produce smaller volumes.

Trade categories

The trade distinguishes several broad categories. Standard labradorite refers to medium-quality material with blue and/or gold schiller. Spectrolite refers specifically to Finnish material and to similar broad-spectrum specimens. Andesine-labradorite, a specific composition transitional toward andesine, has been the subject of some controversy in the trade since the 2000s following the introduction of copper-diffused red and green Chinese material that was initially marketed without disclosure as natural andesine. Rainbow moonstone is a trade name for an unrelated orthoclase variety with adularescence.

Cabochon cutting is the dominant treatment, with the orientation chosen to align the schiller-bearing plane with the cabochon's flat base. Faceted labradorite is occasionally produced from clearer pale specimens but is less commercially significant. Architectural and decorative use, including bookends, table tops and slab cladding, accounts for a meaningful share of total production.

Treatments and synthetic counterpart

Natural labradorite is generally untreated. The schiller cannot be enhanced by any conventional treatment, since the phenomenon depends on the original lamellar exsolution structure. Some Madagascan and Indian material is wax-impregnated to fill surface fractures and improve polish; this is regarded as enhancement rather than fundamental alteration.

The 2000s saw a major controversy concerning red and green copper-diffused andesine-labradorite originally marketed as natural Tibetan or Mongolian material. Subsequent investigation by GIA, AGTA and other bodies established that the material was diffusion-treated, and the trade now requires disclosure of treatment status for any unusual-coloured andesine or labradorite. Mainstream blue and gold labradorite from the established localities is not affected by this issue and is accepted as natural.

There is no commercially significant synthetic labradorite. Glass and other simulants exist but are easily distinguished by hardness, optical character and the absence of true labradorescence.

Use in jewellery

Labradorite has been used in jewellery since at least the early nineteenth century, when its initial European introduction inspired Empire-period and later Romantic-era pieces. Russian and German workshops produced significant labradorite jewellery in the late nineteenth century. Early twentieth-century Arts and Crafts jewellers, notably in Britain and Scandinavia, used labradorite as a primary stone for its iridescent quality. The Finnish design tradition through Lapponia and others has continued labradorite use into the contemporary period.

For modern jewellery the principal questions are setting and orientation. Cabochons are best protected in bezel or half-bezel mounts, given the relatively modest hardness and well-developed cleavage that make labradorite vulnerable to chipping at exposed edges. Faceted material requires careful cutting and is best reserved for protected mounting positions.

Care

Cleaning with warm soapy water and a soft brush is the recommended method. Ultrasonic and steam cleaning are not recommended, since the cleavages and any internal fractures can propagate under vibration. Heat exposure should be avoided. Wax-impregnated material requires particularly gentle handling.