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Image courtesy Greg C Grace | Pink spinel (Left) | Purple spinel (Right)
Image courtesy Greg C Grace | Pink spinel (Left) | Purple spinel (Right)

Colour Investigation: Spinel

Until recent times, spinel was an underappreciated gem with little consumer recognition. However, as demand for ruby alternatives increase, renewed interest and enthusiasm for spinel grows. STACEY LIM reports.

Spinel belongs to the cubic crystal system and forms in a characteristic octahedral crystal shape with well-formed crystals being fairly common in nature. Like diamond and garnet, spinel is singly refractive, showing the same optical properties in all crystal directions.

Spinel offers dramatic and desirable colours, clarity, availability and more affordable prices, all which have contributed to a rekindled desire for this historical gemstone.

Only a small portion of the spinel group of minerals is suitable for use in jewellery. Transparent, gem quality spinels are all close to pure magnesium aluminium oxides with only trace amounts of the other possible substituting elements. A high hardness ranking of 8 to 8.5 on Mohs scale allows this gemstone to be used widely in jewellery designs, as it has been throughout history.

Classified as an allochromatic mineral, spinel’s array of colours result from external influence rather than from elements of its own chemical composition. Colours of spinel include various shades of red, blue and green. It’s also found in brown, brownish-green, black, grey, purple, orange, orange-red and pink.

"Red spinel has famously been mistaken for ruby for thousands of years"

Spinel’s most desirable hue is a deep vivid red, caused by the presence of chromium; the rich red colour rivals that of fine quality ruby. Red spinel has famously been mistaken for ruby for thousands of years. Notable misidentifications include the Black Princes Ruby and the Timur Ruby in the British crown jewels that were both correctly identified as red spinel in the 1950s.

Velvety blue spinel is likened to that of deep Ceylonese sapphire and is a common, affordable alternative. Whilst violet to blue spinel can be coloured by trace amounts of iron, the vibrant blues owe their saturated colour to trace amounts of cobalt.

Steely-grey spinel has become an increasingly popular ‘coloured’ variety as an alternative to black diamond. With eye-clean stones available in larger carat sizes, this newly recognised gem can also be found in a variety of cuts and shapes. With a pale purplish-blue undertone, it is thought the grey colouration may be due to iron.

Intense pinks are adored for their bright attractive colour, caused by chromium, and they reflect a similar tone to vivid fancy-pink sapphire. Orange and purple stones owe their colour to a mixture of iron and chromium.

Spinels are often flawless and free from inclusions, but some inclusions, when they occur, are distinctive. One example is strings of small octahedral crystal inclusions. Whilst treatments are enhancements that rarely occur, the filling of surface cracks in spinels of desirable colour is increasingly being found. Synthetic spinel is common and widely available.

It’s found in Burma, Afghanistan and Sri Lanka and newer deposits in Vietnam and Madagascar have contributed to an increased availability and vast colour range. Most gem spinels are found as rounded pebbles in gem gravels, closely associated with corundum and other alluvial gem materials.

Spinel offers an affordable alternative to its precious counterparts as well as being a magnificent gemstone in its own right. New colours that become available will continue to captivate those on the hunt for a unique and spectacular 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, 12 December, 2018 05:58pm
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