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Image Courtesy: Greg C Grace
Image Courtesy: Greg C Grace

Colour Investigation: Garnet

The spectacular phenomenon of colour change can only be seen in a small handful of gemstones. The garnet group, usually recognised for its rich reds, can supply fine examples of this playful effect. STACEY LIM reports.

Deep crimson garnets have a long history and can be traced back throughout the ages adorning kings, queens, pharaohs and priests. Modern consumers continue to celebrate this group of minerals and the many other coloured varieties that are currently commercially available.

Within the garnet group are six species important to gemmologists; these can be divided into at least 17 varieties. Garnets are rarely completely pure; they usually contain a mix of two or more varieties. Each garnet essentially has the same crystal structure but varies in chemical composition due to the substitution of various elements, and this produces different properties and colours by which the garnet varieties can be identified.

“Colour change garnets are a rare occurrence, with limited deposits in Sri Lanka, Tanzania and Madagascar”

In chemical terms, the most famous garnet varieties are aluminium-rich silicates consisting of pyrope, almandine and spessartine. Typical colours are red, brownish-red, violet-red and orange-red. Although red garnets are the most common and widespread – forming in metamorphic rocks on every continent – this gemstone can be found in a multitude of hues including yellow, orange, pink, purple, brown, black, green and a colour-change variety.

Interestingly, the cause of colour in garnet varies; some species are idiochromatic, that is, they are ‘self-colouring’ and owe their colour to metal oxides inherent in the chemistry of the gemstone, while others are described as allochromatic, coloured by the inclusion of traces of different metal oxides during crystallisation.

In the late 1960s, a new variety of pyrope-spessartine was discovered – Malaia garnet (or Malaya), also known as Umbalite after the Umba Valley in Tanzania. Malaia garnets offer delicate pale pinkish-orange to darker pink-orange shades when coloured by manganese and some iron. The presence of varying amounts of chromium and/or vanadium can produce a colour change that flashes from reddish-purple to a steely blue, green or greyish tone. Other possible colour combinations are reddish-orange to red, greenish-yellow to pinkish-red, light-red to purplish-red and bluish-green to light violet-purple.

Colour change in gemstones 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 light sources, such as a globe or candle, that contain a higher proportion of red wavelengths will cause the gemstone to appear red. As the light source is altered so is the colour of the gemstone in view.

Colour change garnets are a rare occurrence, with limited deposits in Sri Lanka, Tanzania and Madagascar. Fine quality gemstones can produce a strong and attractive red to green colour change, rivalling that of alexandrite, a gemstone so famous for this effect that it is often referred to as the ‘alexandrite effect’.

The garnet group of gemstones is vast, complex and variable, offering so much more than a simple red gemstone.












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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