LABRADORITE A Feldspar Mineral with a Rainbow Inside
Rock&Gem Magazine|June 2021
Collectors often dig labradorite as a colorful rock, but it is actually a mineral, not a rock. It is one of a half dozen varieties of feldspar divided into two groups that make up the crust of the earth. One group is the potassium feldspars, including microcline. The other group is a plagioclase feldspar, including labradorite.
BOB JONES

Labradorite occurs in two forms, massive rock outcrops useful in construction and lapidary work. It is also found in small crystalline and gemmy phenocrysts in volcanic rock suitable for faceted jewelry. It is most often found as huge rock masses that outcrop in areas as part of huge plutonic intrusive formations. These rock-like masses of labradorite often show various colors that flashback at the viewer when the rock is tilted and moved. The crystalline gem phenocrysts are much less common and can be a variety of colors. With a hardness of six to six and one-half, they can be used as faceted gemstones when properly set.

The remarkable play of colors seen in massive labradorite is predominantly blue, but flashes of bronze, golden, red, green and yellow show up as a piece is rotated or moved under a strong light source. Both exciting massive and gemmy types of labradorite can be self-collected. Outcrops of massive labradorite are found in a host of countries, including the United States, where it has been mined as a decorative stone for carvings and less often for jewelry.

CANADIAN DISCOVERY

Massive labradorite was first collected on the Isle of Paul, near Nain, Labrador, Canada, hence the name for this calcium, sodium aluminum silicate. There are other nearby Labrador sources, but the material from near Nain collected in 1770 created quite a stir. The specimens collected showed a remarkable play of colors that excited scientists and triggered a search for more such beauty. It was later found in even more spectacular form in Tvedestrand, Kragero, Norway and surrounding environs. Finland yielded the most spectacular labradorite at Ylamoa, Lammenpa. Some pieces were so colorful as to be called spectrolite. Later, Russia and elsewhere produced equally exceptional labradorite.

What makes massive labradorite so appealing is the play of colors caused by its internal structure. As this feldspar crystallizes, it has a penchant for forming what is called polysynthetic twinning or lamellar twinning. This is referred to as a two-phase razor-thin intergrowth of sodium and calcium feldspar, which alternate such that their structure interferes with and scatters light entering the stone. These alternating microstructures form a regular pattern of closely spaced parallel lamellae that vary slightly in chemical composition, acting like a diffraction grating scattering light that splits into its component colors. Some colors are absorbed, but others are reflected for the viewer to see and enjoy.

This colorful display is often called labradorescence. The surface of the stone flashes broad to narrow bands of color, sometimes soft and iridescent, and sometimes bright and flashy. As you rotate the stone, it changes the angle at which the light strikes the piece making the colors wax and wane and change hues.

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