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Spectrolite (Left) Rainbow Moonstone (Right). Image courtesy: Greg C Grace
Spectrolite (Left) Rainbow Moonstone (Right). Image courtesy: Greg C Grace

Colour Investigation: Labradorite

The unexpected flash of colour and light rolling along the surface of a gemstone can be awe-inspiring. STACEY LIM reports that labradorite captures this spectacular effect known, eponymously, as labradorescence.

This visual phenomenon observed in labradorite is not due to the body colour of the gemstone but rather is a consequence of the diffraction of light from beneath the surface. Typically possessing light to dark grey body colour, labradorite can transform before one’s eyes when a light source is directed at its surface.

Feldspars are the most common minerals of the earth’s crust and whilst for the most part they are lacklustre, labradorite is one variety that is truly extraordinary.

Belonging to the plagioclase series of feldspars, this mineral was named after the Labrador Peninsula in Canada where it was first identified in 1770. Today, Madagascar and Finland are the principle sources of gem-quality labradorite, with Madagascar producing gemstones with an intense blue-violet sheen that are in high demand.

"The value of a gemstone rests on the strength, evenness and colour of the schiller"

Renowned for its labradorescent sheen, called schiller, the source of this playful colour-creating phenomenon is an internal structure composed of lamellar twinning; a delicate layering of alternating sodium-rich (albite) and calcium-rich (anorthite) plagioclase feldspar minerals. As white light penetrates the gemstone’s surface, it reflects off these internal twinning planes producing flashes of colour that can be seen in certain directions and at certain angles.

These spectral colours of light, ranging from red through to violet, are a result of light interference from reflections off the thin layers of feldspar minerals.

The colours produced are dependant on the thickness of the alternating layers. More commonly unearthed with a blue-green schiller, labradorite is traditionally favoured with a rich blue-violet labradorescence. Occasionally exceptional quality specimens may present flashes of the full spectrum of colour and are given the trade name of spectrolite (Figure 1). The value of a gemstone rests on the strength, evenness and colour of the schiller.

Like other feldspars, labradorite can form in large crystalline masses, and the size of gem-quality material is restricted only by the ability to produce a sufficient schiller that is viewable from an appropriate direction. With the possibility of a gemstone parting or splitting along the twin planes of weakness, faceting the gemstone material can be a challenging task.

A transparent-colourless to translucent-white variety of labradorite, found in the Indian state of Bihar, displays a blue through to multi-coloured schiller and has been known in the marketplace as ‘rainbow moonstone’ (Figure 2) since the mid-1990s. True moonstone is actually orthoclase, a potassium feldspar, which obtains its characteristic blue schiller from included layers of albite (a plagioclase feldspar), whereas labradorite obtains its schiller from lamellar twinning of albite and anorthite, two different plagioclase feldspars.

Thus, although moonstone is chemically a different kind of feldspar and its source of schiller is different to that of labradorite, the colour effect is similar enough that the name ‘rainbow moonstone’ has become an accepted name within the industry for this variety of labradorite.












ABOUT THE AUTHOR
Stacey Lim

Contributor • Registered GAA Gemmologist & Valuer


Stacey Lim FGAA BA Design, is a qualified gemmologist and gemmology teacher/assistant. She is a jewellery designer, marketing manager and passionate communicator on gemmology. For information on gemstones, visit: gem.org.au









Wednesday, 23 May, 2018 11:08am
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