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Gemstones














Colour Investigation: Zircon

One of the oldest minerals found in the Earth’s crust, zircon has long being used as a substitute for diamonds. STACEY LIM provides some background.

By calculating the amount of lead produced by the decay of radioactive element traces, scientists have dated zircon crystals found in Western Australia as more than 4.4 billion years old. These impurities were also responsible for the eventual changes in the physical properties of zircon, including its colour.

Zircon’s appealing vibrant blue colour, which is created by heat treatment, is regarded as acceptable because the gemstone remains natural and is not substituted by something else. Apart from its colour, zircon’s key attractions as a gemstone reside in its optical properties. Its high refractive index, second only to diamond, creates a high lustre and its dispersion is almost as great as diamond, giving it considerable ‘fire’.

These properties explain why colourless zircon has been used as a diamond substitute for many years, particularly in the 19th century when the main diamond simulant was glass - and which lacked fire - was soft, and easily damaged.

"Together with its high refractive index, which is between that of zircon and diamond, CZ is also a very efficient diamond alternative."

Because of this, zircon today is really the only natural gemstone that is a relatively convincing diamond substitute. However, it is easily distinguishable from diamond because of its optical properties. Furthermore, while diamond has a cubic crystal structure and only one refractive index, zircon has a tetragonal crystal structure and is doubly refractive.

Its high birefringence also results in a ‘doubling’ of the back facet edges that is usually quite visible with a loupe. Colourless zircon is quite rare and, conceivably, the best current source nowadays may be 29 recycled Victorian-era jewellery.

During the 20th century, other synthetic materials were developed as cheaper substitutes for diamond. The most successful of these, first developed in 1976, is cubic zirconia – or CZ. As the name suggests, CZ has a cubic crystal structure and is singly refracting like diamond.

Importantly, it has a dispersion even greater than diamond and thus considerable fire. Together with its high refractive index, which is between that of zircon and diamond, CZ is also a very efficient diamond alternative.

However, although CZ is now a common diamond simulant it is still quite distinguishable from diamond. Because it is a synthetic substance, CZ is generally flawless, while most diamonds contain natural inclusions or other tiny flaws.

Further, CZ’s greater dispersion results in a ‘showy’ rainbow brilliance that makes it look different to diamond, particularly in larger stones. CZ has a hardness of around 8.5 to 9 on Moh’s scale that is acceptable for a gemstone; however, it scratches more easily than diamond. Everyday wear in an engagement ring, for example, will soon dull its sparkle.

Perhaps because of the similarity in names, cubic zirconia is sometimes confused with natural zircon, which is naturally occurring zirconium silicate, a quite different substance. However, because cubic zirconia is now usually referred to as CZ, this confusion is less common.

Undoubtedly, zircon will continue to be a useful substitute for diamond in the years to come.












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









Monday, 23 July, 2018 12:33am
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