Belomorite

History and name

A decade and a half after the discovery of this albite variety near the White Sea coast, Alexander Fersman described the history of his find in a lyrical essay titled “Belomorite.” He and a companion disembarked at the Arctic Circle station in the Loukhsky District of Karelia and walked toward the “Blue Pale,” a mined-out feldspar vein in a swampy area between hills (varaks) about six kilometers east of the station, near the White Sea shore. In an old working among dark amphibole shales, they found a snow-white albite vein at least ten meters long, rising to a hilltop and branching into shale rocks.

Fersman sat by a stack of feldspar and observed a white, barely bluish stone that was clean and even, with a mysterious light playing on its shiny surfaces—gentle bluish-green iridescences occasionally flashing reddish, but mostly a continuous moonlight-like glow from within. The stone’s delicate pattern of thin stripes crossed it in several directions. Fersman named it “belomorite,” explaining that the White Sea shimmered with moonstone colors or the stone reflected the sea’s pale blue depths. Samples were taken to the Peterhof lapidary factory, recommended as a new jewelry stone.

Properties

The iridescence of belomorite has been studied with generally consistent conclusions, attributing the bluish or greenish glow to specific defects in its layered structure. Fersman noted that the mysterious light played on shiny surfaces along cleavage or fracture lines, and the effect strengthens after polishing. The mineral consists of extremely thin parallel plates; light reflecting from internal cleavage planes undergoes multiple refractions, causing color interference on spinodal decomposition structures comparable to light wavelengths.

This property is not unique, as it characterizes many moonstones. Other iridescent plagioclases, like labradorites, show a narrowly directed colored reflection along the b axis, often in blues and dark blues. Iridescence is a pseudochromatism from light interference on such structures. The optical effect is often called adularescence, involving scattering of white light by submicroscopic defects like microperthite ingrowths or thin cleavage plates.

According to Rayleigh scattering theory, short-wave radiation scatters more, giving reflected light a bluer tint. Belomorite sometimes exhibits weak orange luminescence under ultraviolet rays.

Mineral formation

Belomorite is an igneous mineral found in many granites and granite pegmatites. While albites like oligoclase are common rock-forming minerals worldwide, the iridescent varieties resembling belomorite are less widespread but not truly rare. Moonlight-like plagioclases have long been known from pegmatite veins in the Middle Urals and near Ulan-Ude in Buryatia.

Jewelry-grade belomorite is mainly mined in Sri Lanka, where it is often called Ceylon opal, with substantial deposits also in Madagascar, Tanzania, India, and parts of the United States. Other occurrences exist in Australia, several European nations, Kenya, Norway, and Japan. The type localities are in North Karelia, where Alexander Fersman discovered the stone, and the southern Kola Peninsula.

Usage

Belomorite is an inexpensive and popular ornamental stone used in jewelry as a variety of moonstone. It is typically cut into cabochons, often double-sided and convex to enhance brilliance, unlike labradorite which is cut into flat plates. Its brittleness and perfect cleavage make processing difficult, as the mineral often breaks; thus it is most often cut into simple cabochons (oval, round, teardrop-shaped) or balls, accounting for cracks and cleavage lines. Moonstones are frequently counterfeited with synthetic minerals that have brighter iridescence and are less fragile than natural belomorite.