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Cubic Zirconia Key Facts
- 8.5 on the Mohs scale of hardness
- A man-made colourless stone
- Can be used as a substitute for diamond
General Information on Cubic Zirconia
Cubic zirconia (or CZ) is zirconium oxide (ZrO2), a mineral that is extremely rare in nature but is widely synthesized for use as a diamond simulant. The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. As such it can be used as a diamond substitute in your engagement ring.
It is the oxide of the metallic element zirconium. Zirconium dioxide, known as baddeleyite, is found as a naturally occurring mineral, but with a different, monoclinic, crystalline structure. To induce it to crystallise in the cubic system, it is 'doped' with small additions of other minerals, mainly yttrium or calcium. Cubic zirconia should not be confused with zircon, which is a rare naturally occurring gemstone. Because of its low cost, durability, and close visual likeness to diamond, synthetic cubic zirconia has remained the most gemologically and economically important diamond simulant since 1976. Its main competition as a synthetic gemstone is the more recently cultivated material simulated moissanite.
It is a dense substance with a specific gravity between 5.6-6. Cubic zirconia is relatively hard, at about 8.5 on the Mohs scale - nowhere near diamond, but much harder than most natural gems.
Cubic zircon is colourless whereas only the rarest of diamonds are truly colorless, most having a tinge of yellow or brown to some extent. By comparison, CZ can be made in most cases entirely colourless, equivalent to a perfect "D" on diamond's color grading scale.
The addition of certain metal oxide dopants into the feed powder results in a variety of vibrant colors. For example, the addition of cerium results in yellow, orange or red. Chromium creates green. Neodymium results in purple, erbium creates pink and titanium results in golden brown.
In recent years manufacturers have sought ways of distinguishing their product by supposedly 'improving' cubic zirconia. Coating finished CZs in a film of diamond-like carbon (DLC) or Amorphous Diamond is one such innovation, a process using chemical vapour deposition. The resulting material is purportedly harder, more lustrous and more like diamond overall. The coating is thought to quench the excess fire of CZ, while improving its refractive index, thus bringing it more in line with diamond.
Another technique first applied to quartz and topaz has also been adapted to cubic zirconia: vacuum-sputtering an extremely thin layer of metal oxide (typically gold) onto the finished stones creates an iridescent effect. Unlike DLC, the surreal effect is not permanent, as abrasion easily removes the oxide layer.
Cubic zirconia is so optically close to diamond that only a trained eye can easily differentiate the two. There are a few key features of CZ which distinguish it from diamond, some observable only under the microscope or loupe, for example, dispersion. With a dispersive power greater than diamond (0.060 vs. 0.044) the more prismatic fire of CZ can be seen by even an untrained eye. CZ has an 8.5 to 9.0 on the Mohs hardness scale. CZs are heavyweights in comparison to diamonds; a CZ will weigh about 1.7 times more than a diamond of equivalent size. Obviously, this difference is only useful when examining loose stones. Contemporary production of cubic zirconia is virtually flawless, whereas most diamonds have some sort of defect, be it a feather, included crystal, or perhaps a remnant of an original crystal face (e.g. trigons). CZ has a refractive index of 2.176, compared to a diamond's 2.417. Under close inspection with a loupe, the facet shapes of some CZs appear different from diamonds.
CZ's place as the most realistic simulant ever produced and have made it into one of the best selling 'gemstones'. Unfortunately, because it is inexpensive many manufacturers have churned it out in cheap jewellery, usually in 9 carat gold. To create an engagement ring or other article of jewellery which looks realistically like diamond jewellery, it is necessary to use a real diamond mount, and to have the stones properly set, rather than skimp on the quality of the setting work. If CZ's are mounted in real, high quality diamond mounts, then they are impossible to tell apart from diamonds using the naked eye and without instruments.
Other diamond simulants include glass, often known as 'paste', white sapphire, both natural and synthetic, rock crystal (colourless quartz), spinel, both natural and synthetic, colourless sapphire, both natural and synthetic, YAG (yttrium aluminium garnet), strontium titanate, and the most recent moissanite. One simple test for unmounted CZ is weight; due to its high specific gravity, CZ is about 65% heavier than diamond.
CZ is quite hard, and retains it sparkle and polish well. It should not be allowed to rub against other gemstones, particularly diamond, because this will cause wear and scratching. Wearing two diamond rings next to each other in such a way that the stones can come into contact with each other will cause wear and damage to both stones.
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