Cubic zirconia (or CZ) is the cubic crystalline form of zirconium dioxide (ZrO2). The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It is sometimes erroneously called "cubic zirconium".

Because of its low cost, durability, and close visual likeness to diamond, synthetic cubic zirconia has remained the most gemologically and economically important competitor for diamonds since 1976. It is sometimes marketed under the brand name "Diamonique". Its main competition as a synthetic gemstone is the more recently cultivated material, synthetic moissanite.

History

Discovered in 1892, the yellowish monoclinic mineral baddeleyite is a natural form of zirconium oxide. It has little economic importance because of its rarity.

The extremely high melting point of zirconia (2750°C) makes the controlled growth of single crystals difficult, as no existing crucible could hold the material in its molten state. However, stabilization of cubic zirconium oxide had been realized early on, with the synthetic product stabilized zirconia introduced in 1930. Although cubic, it was in the form of a polycrystalline ceramic: it was used as a refractory material, highly resistant to chemical and thermal (up to 2540°C) attack.

Seven years later, German mineralogists M. V. Stackelberg and K. Chudoba discovered naturally occurring cubic zirconia in the form of microscopic grains included in metamict zircon. This was thought to be a byproduct of the metamictization process, but the two scientists did not think the mineral important enough to give it a formal name. The discovery was confirmed through X-ray diffraction, proving the existence of a natural counterpart to the synthetic product.

As with the majority of grown diamond substitutes, the idea of producing single-crystal cubic zirconia arose in the minds of scientists seeking a new and versatile material for use in lasers and other optical applications. Its production eventually exceeded that of earlier synthetics, such as synthetic strontium titanate, synthetic rutile, YAG (yttrium aluminium garnet) and GGG (gadolinium gallium garnet).

Some of the earliest research into controlled single-crystal growth of cubic zirconia occurred in 1960s France, much work being done by Y. Roulin and R. Collongues. This technique involved molten zirconia being contained within a thin shell of still-solid zirconia, with crystal growth from the melt: The process was named cold crucible, an allusion to the system of water cooling used. Though promising, these attempts yielded only small crystals.

Later, Soviet scientists under V. V. Osiko at the Lebedev Physical Institute in Moscow perfected the technique, which was then named skull crucible (an allusion either to the shape of the water-cooled container or to the form of crystals sometimes grown). They named the jewel Fianit after the institutes name FIAN (Physical Institute of the Academy of Science), but the name was not used outside of the USSR. Their breakthrough was published in 1973, and commercial production began in 1976. By 1980 annual global production had reached 50 million carats (10 tonnes).

Innovations

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 vapor 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 making it appear more like diamond. Additionally, because of the high percentage of diamond bonds in the amorphous diamond coating, the finished simulant will show a positive diamond signature under Raman spectroscopy.

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) on to the finished stones creates an iridescent effect. This material is marketed as "mystic" by many dealers. Unlike DLC, the surreal effect is not permanent, as abrasion easily removes the oxide layer.

THE MINERAL SPINEL

Chemistry: MgAl2O4 , Magnesium Aluminum Oxide
Class: Oxides
Group: Spinel
Uses: as a gemstone and as a mineral specimen
The physical properties of spinel.
Spinel is a very attractive and historically important gemstone mineral. Its typical red color, although pinker, rivals the color of ruby. In fact, many rubies, of notable fame belonging to crown jewel collections, were found to actually be spinels. Perhaps the greatest mistake is the Black Prince's Ruby set in the British Imperial State Crown. Whether these mistakes were accidents or clever substitutions of precious rubies for the less valuable spinels by risk taking jewelers, history is unclear. The misidentification is meaningless in terms of the value of these gems for even spinel carries a considerable amount of worth and these stones are priceless based on their history, let alone their carat weight and pedigree.

Today, expensive rubies are still substituted for by spinel in much the same way a diamond is substituted by cubic zirconia. Not to commit a fraud or theft but to prevent one. Spinel may take the place of a ruby that would have been displayed in public by an owner who is insecure about the rubies safety. The spinel probably is still valuable but better to lose a $100,000 dollar spinel than a $1 million dollar ruby!

Spinel and ruby are chemically similar. Spinel is magnesium aluminum oxide and ruby is aluminum oxide. This is probably why the two are similar in a few properties. Not suprisingly, the red coloring agent in both gems is the same element, chromium. Spinel and Ruby also have similar luster (refractive index), density and hardness. Although ruby is considerably harder (9) than spinel, spinel's hardness (7.5 - 8) still makes it one of the hardest minerals in nature.

Spinel may be the poorer cousin of ruby, but its pinker color and other qualities make it attractive in its own right. Spinel typically forms in well formed octahedrons. But it is famous for a type of twinning that bears its name, the Spinel Twin Law. Spinel Law twinning is also found in other isometric minerals such as diamond, galena, cristobalite , magnetite , franklinite and other members of the Spinel group. This type of twinning produces a twin plane that is parallel to one of the octahedral faces. The plane acts as a mirror plane and produces a left and right side that are mirror images of each other. This may not sound all that spectacular for a very symmetrical mineral like spinel which is loaded with mirror planes. However this mirror plane is not parallel to any of the others and actually lowers the symmetry of the crystal (only in appearances though).

A good description of the twin is hard to explain, but here it goes. The plane falls (of course) in the center of the crystal, dividing it in half. The two octahedron faces parallel to the twin plane are equilateral triangles. Each point of the triangles is doubled across the twin plane with an indentation between them. The crystal looks like it has trigonal symmetry, but the three indentations are a clue that this crystal is a twin. Twins of spinel are rare, but their popularity makes them readily available on the market.

PHYSICAL CHARACTERISTICS:

Color is red, green, blue, purple, brown, and black; but red is by far the more common color.
Luster is vitreous.
Transparency: Crystals are transparent to translucent and sometimes nearly opaque.
Crystal System: Isometric; 4/m bar 3 2/m
Crystal Habits include the typical octahedron, but can be found as dodecahedrons and combinations of other isometric forms. Also as rounded grains in alluvial placer deposits.
Cleavage: None
Fracture is conchoidal.
Hardness is 7.5-8.0
Specific Gravity is 3.6-4.0
Streak is white.
Other Characteristics: index of refraction is approximately 1.71 - 1.76 and rutile inclusions may produce six or four rayed stars or asterisms.
Associated Minerals include Calcite, dolomite, corundum and garnets .
Notable Occurrences include Burma, Sri Lanka, Brazil and Afghanistan.
Best Field Indicators are twinned crystals if present, color, hardness, density and locality.
Spinel: collector's favourite

Spinel is the great impostor of gemstone history: many famous rubies in crown jewels around the world are actually spinels. The most famous is the Black Prince's ruby, a magnificent 170-carat red spinel that now adorns the Imperial State Crown of England in the British Crown Jewels after a long history: Henry V even wore it on his battle helmet! The Timur ruby, a 361-carat red spinel now owned by Queen Elizabeth, has the names of some of the Mughal emperors who previously owned it engraved on its face: an undeniable pedigree!

In Burma (now known as Myanmar), where some of the most beautiful colours are mined, spinel was recognised as a separate gem species as early as 1587. In other countries the masquerade went on for hundreds of years. Spinels were most often referred to as 'balas rubies', which may have referred to their colour or their country of origin.

Now treasured for its own sake, spinel is a favourite of gem dealers and collectors on account of its brilliance, hardness and wide range of spectacular colours. In addition to beautiful rich reds, spinel can be found in a range of gorgeous pastel shades of pink and purple. Of particular interest is a vivid hot pink with a tinge of orange mined in Burma. It is one of the most spectacular gemstone colours seen in any species at all. Spinel also comes in beautiful blue tones called cobalt spinel, but these are very, very rare.

Because spinels made in a laboratory are often used for imitation birthstone rings, many people think 'synthetic' when they hear the name spinel. Often enough, they have never even seen the real thing.

In fact, the main factor preventing the spinel from achieving greater recognition is its rarity. Fine spinels are now rarer than the rubies they used to imitate. Strangely, they are also more affordable: in the gem world, being too rare can be a drawback because so few people even get a chance to develop a fondness for the varieties in question.

In addition to Burma, spinel is mined in Sri Lanka, Tanzania, and Tadzhikistan, part of the former Soviet Union.

Spinel is a durable gemstone that is perfect for all jewellery uses. It is most often faceted in oval, round, or cushion shapes and is not currently found in calibrated sizes due to its rarity.