The most important of the gem families, corundum provides us with two of the best-known stones: ruby and sapphire. The popularity and price of these gemstones remain strong due to the beauty, durability and versatility of the material. Unfortunately, however, the ongoing development of synthetics, imitations and enhancement treatments has made the task of choosing and buying sapphire or ruby complex and sometimes risky.
jewelry makers and buyers should assume that the vast majority of corundum they see is enhanced in some way. Since the 1960s, heat treatment has been a common practice. Sadly for those on a budget, natural untreated rubies and sapphires carry a significant price premium: in terms of cost and rarity, a natural Burmese ruby of 10 carats is now on a par with natural coloured diamonds.
The colour zoning commonly seen in corundum relates to the growth layers of a crystal, and appears as a series of concentric hexagons parallel to the prismatic crystal faces. Ruby exhibits strong pleochroism, showing yellow-red and deep carmine red when viewed from different angles; this optical effect can also occur in sapphire. Corundum can be transparent to opaque and have a vitreous to dull, greasy lustre. When transparent, the material is moderately brilliant.
- Sapphires and rubies are priced according to colour, clarity, size, cut and proportions. When pricing blue sapphires the purity and intensity of blue is critical. Ideally, it should possess a violet overtone with no sign of grey or green. The lightness or darkness of a stone, colour zoning and clarity all need to be considered; irregular colour zoning detracts from a stone’s value and beauty. Inclusions are not necessarily bad – they can indicate whether or not the stone has been treated or identify its origin. Larger gems are more expensive, but small stones with good colour and clarity can still have a reasonable value.
- Rubies should be viewed in different light conditions as the spotlights in a shop make the colour intense and “hot”, while daylight cools the colour down. Dark stones can appear nearly black in incandescent light.
- Origin has a direct impact on price, so make sure your receipt states the country of origin, especially if the gem is Burmese, If you are planning to buy an expensive natural ruby or sapphire, it is important to obtain a laboratory report before spending your money – the majority of reputable gem dealers would agree to this.
- Colourless sapphire is sometimes used as a cost-effective alternative to diamond; if cut well the stones can be brilliant and even contain some fire. It is a tougher stone at small sizes than white topaz and quartz, which have a tendency to break when being gypsy set.
Working with Corundum
- Most large, fine sapphires are oval, emerald or cushion cut – the smaller stones have a greater variety of shape. You should examine the cut of rubies and sapphires carefully. Stones that have been cut deep to retain weight can be difficult to set in jewelry and should cost less per carat because they are overweight for their size. A shallow cut will sacrifice brilliance for lightness of colour and the gemstone will contain a noticeable “window”. Corundum has to be repolished following heat treatment to remove the damaged exterior layer and, if this is not done with care, the gemstones can end up with a double girdle and/or pockmarks. Rubies and sapphires are often recut in the West to correct proportions, but the subsequent loss of weight and size will result in an increase in the carat price of the stone.
- Corundum is a durable gemstone – it is the hardest material after diamond and is used as an industrial abrasive. However, although it has no cleavage, it does have preferred directions of parting, which means that the material can be brittle despite its hardness value. It has a conchoidal fracture, and the many internal flaws or cracks found in rubies and sapphires can weaken a stone. The frequent twinning that occurs with ruby crystals can also make the material liable to damage. These factors have to be taken into account when cutting, polishing and setting corundum. However, ultrasonic and steam cleaners will not harm the stones.
- The hardness of corundum does give the gemstones longevity, making them ideal for jewelry that will take a lot of wear and tear over many years. Rubies and sapphires might suffer some scratches or lose a little sharpness on the facet edges over time, but the metal work is more likely to fail first.
Prongs and settings loosen up with continual use and the tips of claws can become brittle, crack or wear down. To prevent the loss of an expensive sapphire or ruby, check the stone for movement and inspect the settings from time to time.
Prior to the 1800s, red spinel, red garnet and ruby were all thought to be one gemstone (ruby) because of the similarity of colour. Thus a variety of misnomers appeared: almandine ruby (garnet), Australian ruby (garnet), balas ruby (sinel), Bohemian ruby (garnet) and Cape ruby (garnet).
Composite doublets are manufactured by gluing a pale or colourless corundum crown to a synthetic blue sapphire pavilion. Alternatively, cobalt blue glass is given a crown of garnet, or a low-grade sapphire crown and pavilion are simply glued together with a coloured epoxy to create a sapphire of an expensive colour. Numerous doublets are sold to travellers in the Far East; they are difficult to spot when set in jewelry and do not always show up in gemmological tests. Blue star sapphires can also be manufactured: a star rose quartz often has blue enamel painted on its flat base, or a six-ray star may be engraved on the back of a synthetic carbochon.
- There are several ways in which the colour of corundum is enhanced. One is fracture filling, a common practice in the industry that reduce the visibility of flaws by making the fractures nonreflective. The colour of the gem is not diminished and the white patches around the fractures are lost. Oil, wax, paraffin, glass and epoxy resin are all used as fillers.
- Heating corundum lightens or intensifies colour, improves uniformity and enhances clarity by melting some of the silk that commonly occurs. The process is permanent. The inclusions may show evidence of heat treatment, as the heat can cause small crystals to melt or change, but if there are no inclusions it is difficult to tell whether the gemstone is natural or not. Irradiation is occasionally performed on sapphires, but is not an accepted treatment as the colour is not permanent and will fade.
- There has been much discussion over diffusion treatment. Unlike traditional heat treatment, this colours the surface of the gemstone only, while the centre remains colourless. The procedure involves introducing chemicals (the colouring agents of ruby and sapphire) into the upper surface of a colourless or pale stone and then heating it over a prolonged period of time. The effect is permanent, but the surface colour could be removed if the stone were badly chipped and needed repolishing. However, it is possible to repeat the process and restore the colour. The practice is acceptable as long as it is disclosed and the price of the gemstone reflects the fact that it has been treated. The diffusion treatment can also induce asterism in sapphires and rubies.
- Sapphires and rubies are also diffusion treated in the presence of beryllium. Low-grade corundum is heated alongside chrysoberyl to achieve intense orange and padparadscha colours, while Vietnamese and Tanzanian (Songea) rubies that normally have the colour of red garnet are beryllium treated to bright red-orange colours. However, the beryllium diffusion treatment doesn’t always work and some stones end up with purple-black blotches (or explosions) within the material. There has been tremendous controversy regarding this process, with many fearing it would affect the sale of natural rubies and sapphires, but that has not been the case. Instead, nontreated stones have become more desirable and valuable, Standard gemmological tests cannot detect this type of treatment, which requires microscopic analysis of the inclusions. It is proposed that these stones should be labelled as “heat treated with additives (including beryllium-bearing substances)”, and that there should be a warning that the resulting colour could be removed if the stone is damaged or repolished. There is disturbing evidence that suggests the beryllium treatment poses a health hazard to the workers who process and cut the gemstones and to the merchants who handle them.
In the early 20th century the market was flooded with synthetic rubies, causing widespread apprehension as sales of natural rubies decreased and their value dropped. The synthetic rubies were manufactured by the Vernauil method, in which powdered aluminium oxide and colouring agents were exposed to a flame. A synthetic colourless sapphire, called diamondite, was also produced this way.
Nowadays, highly developed flux-grown synthetics are manufactured for gem use, and sell in four inclusion-based grades and several size or weight categories. These Chatham and Ramaura rubies and sapphires are far from cheap. Synthetic star rubies and sapphires, called Linde stars, have been sold since 1947 and have been extremely popular. Synthetic colour change sapphires are also manufactured.