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Image courtesy Hamid Bros Gem Merchants
Image courtesy Hamid Bros Gem Merchants

Colour Investigation: Alexandrite

Emerald by day and ruby by night, alexandrite is perhaps the best example of the magical colour-changing variety of gemstones. STACEY LIM reports.

Awe and wonder surround the spectacular phenomenon of colour change in gemstones. The fascination of seeing colours shift before one’s eyes can leave consumers speechless with utter disbelief.

Alexandrite, a variety of the gemstone chrysoberyl, is a fine example of this magical effect, with superior quality material producing a dramatic colour change that is sometimes described as emerald by day and ruby by night. This is not surprising as the ‘magician’ behind the scenes in this extraordinary colour play is none other than chromium, the element responsible for the rich red of some of the world’s best quality rubies and, remarkably, the vivid green seen in emeralds.

Sometimes referred to as the alexandrite effect, this colour change is noted in a small handful of other gemstones, although not to the intensity that can be seen in alexandrite.

Alexandrite was first discovered in the Ural Mountains in the 1830s but current sources of the gemstone include Sri Lanka, East Africa, India and Brazil. Named alexandrite in honour of Alexander II, the Tsar of the Russian Empire who took power in 1855, this red and green gemstone caught the country’s attention because its colours mirrored the national military colours of Imperial Russia.

Described as an idiochromatic mineral due to its self-colouring properties, chrysoberyl owes its various hues to the mineral’s chemical composition. Alexandrite is a relatively scarce colour-change variety of the chrysoberyl family that contains traces of chromium +3 ions. These ions react to light and absorb specific parts of the light spectrum giving the resulting colour. In alexandrite, the light absorbed by these chromium ions has a wavelength between red and green, and the colour change is dependent on the source of the incident light.

Natural daylight or fluorescent light contains higher proportions of blue and green wavelengths and will cause the gemstone to appear to be green whereas incandescent lighting, such as a globe that contains a higher proportion of red wavelengths, will cause the gemstone to appear red. High-quality alexandrite displays green to bluish green in daylight and a red to purplish red in incandescent light. Alexandrite usually has few inclusions so the value of any alexandrite example rests upon the intensity of its body colour as well as the impressiveness of its colour-changing properties.

The attractive gemstones produced from Russian deposits in the 19th century are still considered to be the most distinctive, displaying vivid hues and bold colour changes. Modern sources of alexandrite tend to exhibit muddier tones with a less-precise colour change.

This mineral’s light-absorbing quality doesn’t end with colour change; alexandrite also exhibits strong trichroism, showing different colours when viewed from different directions. Typical pleochroic colours are deep red/brownish-green or orange-yellow/green and, although they affect the body colour of alexandrite, this attribute has no effect on colour change.

A mysterious mineral with extraordinary qualities, alexandrite’s intriguing colour play continues to pique the interest of collectors, gemmologists and jewellery lovers.












ABOUT THE AUTHOR
Stacey Lim

Contributor •


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









Saturday, 19 August, 2017 04:20am
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