Anorthosite is a coarse-grained, plutonic igneous rock composed almost entirely — typically more than 90 per cent by volume — of plagioclase feldspar, most commonly labradorite or anorthite. It is not itself a gemstone in the conventional sense, but it occupies an important place in gemmology as the primary host rock for gem-quality labradorite that exhibits the vivid iridescent phenomenon known as labradorescence. When the plagioclase crystals within an anorthosite body are of sufficient size, optical homogeneity, and structural regularity to produce strong schiller, slabs and cabochons cut directly from the rock — rather than from isolated crystals — enter the ornamental stone trade. Understanding anorthosite is therefore inseparable from understanding where fine labradorite comes from and why it behaves as it does.

Mineralogy and Composition

The plagioclase feldspars form a continuous solid-solution series between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈). Anorthosite is defined by the overwhelming dominance of this mineral group, with accessory phases — pyroxene, olivine, ilmenite, magnetite — typically constituting less than ten per cent of the rock. The specific plagioclase composition within any given anorthosite body varies with geological setting: Proterozoic massif-type anorthosites, which are the most commercially significant for gem material, tend to contain labradorite (roughly An₅₀–An₇₀ in the anorthite component), whereas anorthosites associated with layered mafic intrusions may carry more calcic compositions approaching pure anorthite.

The crystal grain size is characteristically coarse, with individual plagioclase crystals commonly ranging from one centimetre to several tens of centimetres across. This megacrystic texture is a direct consequence of slow cooling at depth and is precisely what allows the development of the thin, compositionally alternating lamellae responsible for labradorescence. The optical phenomenon arises from light interference between submicroscopic layers of differing refractive index that exsolve within the feldspar during cooling — a process analogous to, though structurally distinct from, the adularescence seen in moonstone.

Geological Setting and Formation

Anorthosite occurs in two principal geological contexts. The first and gemmologically more important is the Proterozoic massif-type anorthosite, enormous plutonic bodies emplaced between roughly 1.0 and 1.8 billion years ago, predominantly within Precambrian cratons. These massifs — some covering thousands of square kilometres — formed through the ascent and fractional crystallisation of mantle-derived magmas under conditions that are still debated among petrologists, but that clearly favoured the buoyant rise and accumulation of plagioclase. The Adirondack Mountains of New York State, the Grenville Province of eastern Canada, Scandinavia, and large portions of southern Africa and Madagascar host classic massif anorthosites of this age.

The second context is layered mafic intrusions such as the Bushveld Complex of South Africa or the Stillwater Complex of Montana, where anorthosite forms discrete layers within a broader sequence of cumulate rocks. These bodies are of greater economic interest for platinum-group elements and chromite than for gem feldspar.

Anorthosite is also the dominant rock type of the lunar highlands — the pale, heavily cratered terrain visible to the naked eye on the Moon. Apollo mission samples confirmed that the lunar crust is composed largely of ferroan anorthosite, crystallised from a primordial magma ocean approximately 4.4 billion years ago. This makes anorthosite one of the oldest and most cosmologically significant rock types known, a fact that lends the material a certain scientific gravitas beyond its ornamental applications.

Gem-Quality Labradorite within Anorthosite

The localities most celebrated for ornamental anorthosite and gem labradorite are closely linked to Proterozoic massif terrains.

• Labrador, Canada: The Nain Plutonic Suite of the Labrador coast, particularly Paul's Island near Nain, has been a source of labradorite since the late eighteenth century, when Moravian missionaries brought the stone to European attention. The material from this region — sometimes marketed as spectrolite in the broader trade, though that name is more precisely applied to Finnish material — displays blue and green schiller of considerable intensity. The host rock is a classic Proterozoic massif anorthosite.
• Madagascar: Large-scale deposits in the Tuléar and Fianarantsoa regions yield anorthosite carrying labradorite with a broad spectral range, including gold, orange, red, and violet schiller in addition to the more common blues and greens. Madagascan material has dominated the commercial market for polished labradorite slabs and cabochons since the 1990s.
• Finland: The Ylämaa deposit in south-eastern Finland produces what is correctly termed spectrolite — labradorite within anorthosite noted for an exceptionally full spectral display, often showing the complete visible spectrum within a single stone. Finnish spectrolite is generally considered the finest material for collector-grade specimens and high jewellery.
• Ukraine: Deposits in the Ukrainian Shield, particularly near Zhytomyr, have supplied labradorite — historically called larvikite in some European trade usage, though that term is more accurately reserved for a Norwegian syenite — for architectural and ornamental purposes for well over a century.

Anorthosite as an Ornamental Material

When anorthosite exhibits pervasive, strongly coloured labradorescence throughout the rock mass, it is polished and sold as an ornamental stone in its own right. Flat slabs are used in architectural applications — flooring, wall cladding, countertops — and smaller pieces are fashioned into cabochons, beads, and carvings. The material is relatively soft by gemstone standards, with a Mohs hardness of approximately 6 to 6.5 inherited from its plagioclase composition, and it cleaves in two directions at roughly 90 degrees, which requires care during cutting and setting.

In the trade, polished anorthosite is rarely distinguished by its rock name; it is sold simply as labradorite, spectrolite, or occasionally under regional trade names. The distinction between a cabochon cut from a single large plagioclase crystal and one cut from a fine-grained anorthosite matrix is largely academic from a commercial standpoint, though the former may show a more uniform and concentrated schiller patch while the latter can display a more complex, multi-directional play of colour arising from the interlocking of multiple crystal orientations.

Treatments and Enhancements

Anorthosite-derived labradorite is not routinely treated in the manner of many coloured gemstones. The labradorescence is an intrinsic structural phenomenon and cannot meaningfully be enhanced by heat, irradiation, or fracture filling. Surface coatings have occasionally been applied to low-grade material to simulate or intensify schiller, but such treatments are readily detected under magnification and are not considered acceptable practice in the reputable trade. Buyers of fine labradorite from established sources can generally assume the material is untreated.

Gemmological Identification

Anorthosite and its constituent labradorite are identified by a combination of properties: refractive indices in the range of approximately 1.559 to 1.568 (varying with anorthite content), a specific gravity of roughly 2.69 to 2.72, and the characteristic labradorescence visible under diffuse lighting. Spectroscopic examination reveals no strong absorption features of diagnostic value; identification rests primarily on physical constants and the optical phenomenon itself. Inclusions in polished material often include fine ilmenite needles, pyroxene grains, and the lamellar twinning planes (most commonly albite and pericline twinning) that are responsible for the schiller.

Further Reading

• GIA — Labradorite and Feldspar Gems (gia.edu)
• International Gem Society — Labradorite (gemsociety.org)