Zircon Vs Cubic Zirconia - are they the same?
The simple answer is NO. They are two entirely different things. Whilst zircon is a naturally occurring gemstone in its own right, cubic zirconia as we know it today, (commonly known as CZ) is a man-made diamond imitation. A few, extremely small natural crystals of Zirconium dioxide were found in the 1930’s but all CZ used in jewellery today is man-made.
Zircon
The zircon crystal is formed from zirconium silicate and it is a source of the metal zirconium. Zircons are mined in Sri Lanka and Australia in addition to Myanmar, Vietnam and Cambodia.
Zircon is found in many colours: blue, white, red, brown, orange, pink, purple and green. It has a very high lustre – adamantine, which is the same as diamond (lustre is the measure of how light reflects from the surface of the gem).
Zircon is a doubly refractive gemstone which means that when a ray of light enters the stone, not only does the light change direction but it also splits into two rays of light that travel through the gem at 90° to each other, these are ‘plane polarised rays’. The amount that the light rays split and refract by is so pronounced in zircon, that it is often possible to see it if you look through the cut stone down through the table facet (the top, flat surface of the gemstone). You will find that where each facet edge is, that you are able to see two parallel edges due to the high double refraction.
Zircon is the oldest mineral on earth. Some that has been found in Australia is 4.4 billion years old. The trace elements in the zircon crystal provide a time scale which can give information about early earth.
The most prized colours of zircon are blue, bright red and green. It is rare to find zircons in large sizes although you can find heavy stones as the density of zircon is far greater than that in other gems of the same size. Much zircon is classed as ‘eye clean’ and as such, gems that do display signs of inclusions are less valuable.
Almost all blue zircon seen in jewellery is heat treated. It usually starts as reddish or brownish crystals and the blue colour is obtained when it is heated at high temperature without any oxygen present.
Zircon crystal, 30.25 carats. From CAME in the Usambara Mountains in Tanzania. Photographs courtesy of http://gemsofeastafrica.com
For many centuries there was confusion between diamond and zircon due to the brilliance and high dispersion you get with zircons. Today, however, we can easily recognise zircon by its double refraction and the wear to the facet edges as it is not as hardwearing as diamond.
Taking into account the wear that occurs to zircon when it is set in jewellery, it is advised that they should be removed when undertaking rough activities such as sports, gardening and housework. To clean your zircon jewellery, use only a very soft brush and warm soapy water. DO NOT subject it to ultrasonic or steam cleaning is this is highly likely to damage the gemstone.
Cubic Zirconia - CZ
Usually colourless and manufactured as a diamond simulant, CZ is now found in colourful varieties too. It should NOT be confused with man made synthetic diamonds which are another completely different topic!
Although, like diamond, CZ is a singly refractive material, there are many other ways that they differ. For example, diamond is the hardest known substance with a score of 10 on Mohs’ scale of Hardness whereas CZ only comes in at 8 – 8.5 meaning that is is not as resistant to scratching as a diamond is. This lower hardness also shows where the facet edges are concerned as with a diamond they are very sharp but with a CZ they can have a tendency to be softer looking. The girdles of diamonds are rarely polished due to their hardness and are either left ‘bruted’ (rough) or have been faceted whereas most CZ girdles will be polished.
Diamond has a high dispersion (the way it scatters the white light as it exits the gemstone). However, CZ has an even higher dispersion and this alone can be an indicator that all may not be as it seems.
Cubic zirconia is produced using the “Skull Melt” process. This method is used because of the extremely high temperature needed to produce the product. It is only used for the production of Cubic Zirconia.
In simplified terms, to make the cubic zirconia: firstly Zirconia Oxide powder and zirconium metal is placed inside the “skull” crucible. The metal is necessary as the powder does not conduct electricity. The metal melts and oxidises and mixes with the zirconium oxide. Around the outside of the skull is a cool water jacket to stop the container melting as the zirconia oxide has to reach very high temperatures. The radio frequency coils provide the heat needed to melt the mixture. The outer edges of the mixture remain cooler and form a crust. Gradually crystals form from the bottom of the mixture and eventually they grow to take up all the space and the solidified mix is lowered out of the skull to retrieve the crystals. During the melting process, the zirconia oxide passes through different crystal systems as it is what is termed a ‘polymorphous’ substance. It does not form in the cubic crystal system (that needed to imitate diamond) until it reaches 2600°C. Stabilisers such as Yttrium Oxide are added to make the crystal stay in its cubic form. After the mixture has cooled the outer shell is removed allowing the crystals to be collected ready to be cut and faceted and used in jewellery.
