Labradorite is a mineral with a hardness of 6 out of 10 on the Mohs scale of mineral hardness [?]. These Triclinicly structured gems are made of sodium calcium aluminosilicate, their full chemical compound being (Na,Ca)(Al,Si)4O8.
Labradorite is a sodium-rich plagioclase feldspar which displays a particular type of iridescence on a dark ground. Plagioclase feldspars are rock-forming, calcium-sodium minerals which form a continuous series ranging from albite, through oligoclase, andesine, labradorite, and bytownite to anorthite. Precise classification is generally not possible in hand specimens, and their physical properties only vary according to calcium content, but labradorite in particular shows a beautiful play of colors.
The iridescent effect of labradorite is probably due to the presence of very fine platelets of different compositions and minute inclusions of ilmenite, rutile, and perhaps, magnetite, which cause refraction.
The ground color of labradorite is a dark smoke gray, but when light strikes it in a particular direction, it displays striking rainbow-colored reflections and lustrous metallic tints (violet, blue, green, yellow, even orange and reddish) known as labradorescence. Specimens with the complete spectrum are most appreciated. The background color is uninteresting, and it is the strength of the labradorescence that gives the stone its value.
It occurs as free-standing tabular crystals, and rare, often twinned, in large masses, with parallel or criss-cross twinning striations. These minerals are always light, and sensitive to pressure. It is soluble in acids, when powdered.
Labradorite is found in igneous rocks, both plutonic and volcanic. It is rare in granitic rocks. It is typical of some eruptive rocks (anorthosites) and metamorphic rocks in Norway, Labrador (Canada) and the Soviet Republics.
The normal rock-like labradorite is also found at other localities in Newfoundland, along the shore of Lake Huron, at Cape Mahul, at Abercrombie and at Morin in Quebec Canada. In Russia it occurs in the Ukraine especially at Gorodishch in the Zhitomir district, and in the Ural Mountains. In the USA small quantities occur in Arkansas, New Mexico and Vermont.
The Malagasy Republic produces a labradorite-moonstone with strong blue labradorescence. Colorless and yellow-brown labradorites have come from New South Wales (Australia). Blue 'flashing' labradorite from India is reported to show a similar effect as the light source, relative to the stone, is moved.
The labradorite used in gems comes mainly from Canada and Finland. Gem-quality material has also been found in Arizona, California - Mode County (sunstone), New Mexico, Nevada and Utah. Similar material comes from Mexico and Australia, Hogarth Range, New South Wales, and near Springsure, Queensland.
Varieties of labradorite include Sunstone, Sunspar, which shows hues ranging from colourless to light yellow on the smaller stones to champagne or straw yellow on the largest; Black moonstone, which is colorless labradorite that shows some chatoyancy when appropriately cut; Rainbow moonstone, which exhibits multi-coloured sheen; Bull's eye, applied to dark labradorite (anorthosite); Lynx eye, labradorite with green iridescence; Opaline feldspar, labradorite from anorthosites; Ox-eye, labradorite that exhibits dark reddish hues.
The name Labradorite derives from its main source, the Isle of Paul, Nain, Labrador, in Canada, where it was first found in 1770 by a Moravian missionary.
At the beginning of the 1940s, labradorite deposits were discovered near Ylijarvi in Karelia (Finland) which exhibit the spectral color particularly well. They are offered in the market as Spectrolite.
Labradorite is cut into gems, or small, not too convex, polished plaques for setting. It is also used as an ornamental material for carving and engraving, for bead necklaces, brooches, rings and ornamental objects.
Its value is quite low, partly because it is hard to use. Few types of work can bring out its characteristic colors.
There is an ornamental material, used in slabs and consisting of a rock containing large pieces of potassic feldspar, which looks similar to labradorite. This material, called larvikite, after the place it was found in Norway, is used for building purposes only. If necessary, labradorite and larvikite could be distinguished by their densities.