Sapphire - The Popular One
When we hear the word sapphire, we generally think of the cool blue hues of the deep ocean. However, the term sapphire is used for all varieties of the gem species corundum that don’t fall into ruby’s colour range. Generally, the word sapphire when used alone refers to the blue variety. The array of other colour variations, such as pink, yellow, padparadscha (orangish pink) or green, are referred to collectively as fancy sapphires, but their hue is stated beforehand when referred to individually.
Of the multitude of sapphire colours available, blue is by far the most valuable. Pinks and padparadscha follow behind blue, while yellows and oranges lie somewhere in the middle. Green, purple and colourless, despite their beauty are generally not sought out in their finer versions, but that does mean that some great deals can be found in these colours!
If ruby is the darling of the gem world, then sapphire, with its range of cool blues is the popular one. In the US especially, more people buy fine jewellery set with sapphires, than with any other coloured gemstone. The engagement ring given to Kate Middleton by Prince William has a beautiful sapphire surrounded by white diamonds. The same ring was previously given to Lady Diana by Prince Charles, with that kind of endorsement, is there any wonder why it’s such a popular gemstone.
Mineral Species: Corundum
Refractive index: 1.76 – 1.77
Specific gravity: 3.9 – 4.1
Colour range: All other colours excluding ruby
Chemical composition: Al2O3
Crystal Structure: Trigonal
Typical cutting style: Oval and cushion mixed cut
Romance and Lore
Although it is difficult to find historical references to when sapphires were first used for adornment, it is generally considered that Sri Lanka was the earliest source, where alluvial rounded pebbles of corundum were mined on a small scale. In his book, Marco Polo wrote of his visit in 1292 “I want you to understand that the island of Ceylon is, for its size, the finest island in the world, and from its streams comes rubies, sapphires, topazes, amethyst and garnet.” Of course, Marco Polo was known to perhaps exaggerate his tales a little, but Greek merchants were also known to visit the island to trade in precious stones as well as tortoise shell and ivory during the reign of Alexander the Great (335-323BC).
From its ability to attract wealth or inspire faithfulness, rumours have been peddled for centuries in an attempt to increase desire for this beautiful gemstone - as I often press upon my students, a good product still needs an inspiring back story! Ancient tales asserted the stone inspired nobility truth and faithfulness, and supposedly protected the wearer from envy and harm. For all their romance, these tales most likely stem from some slightly dubious, ancient sales person trying to offload his wares….. not unlike today! Interestingly, they were also said to protect chastity – although who’s chastity, they are not entirely clear on.
Modern day marketers have named sapphire as the birthstone for September as well as a token of love for a 45th wedding anniversary, noteworthy for those working in sales 😉.
Causes of Colour
It was long suspected in India (but totally unknown to the West) that rubies and sapphires were not only related, but actually the same species of gemstone (corundum), the only difference being minute quantities of different colour causing elements. However, this fact wasn’t scientifically proven until the late 18th Century with the invention of modern gem testing methods. Whilst chromium causes the red in ruby and pink sapphires, a combination of iron and titanium is responsible for the blue colour in sapphire.
In small sizes the price of good quality diamonds far exceeds that of sapphires, for example one popular jewellery wholesale supply company sells a single 2mm VS / GHI diamond for $28.35 whilst a 2mm ruby of excellent quality is sold for only $9 – bargain!
In large sizes however, their prices vary and they can beat the prices of colourless diamonds of excellent quality. Sapphires cannot compete with the prices reached for fine rubies or coloured diamonds, for the simple reason that they aren’t as rare. There is no denying the beauty of a fine sapphire, but rarity plays a major role in determining the value of all gemstones.
As with most coloured stones, the primary consideration when buying a sapphire is its colour. The purer the blue, the more valuable the stone. Any modifying colours generally detract from the value of a pure blue hue. Greenish blue stones are predominantly considered less valuable than slightly purplish blue stones.
To be of ideal colour, it should be neither too dark, nor too light. If the tone (degree of lightness or darkness) is too dark, it has a negative effect on the stone’s brightness and the gem can appear inky or black (similar to many Australian sapphires).
In terms of fancy coloured sapphires, their value can rarely match that of fine blues. Next in terms of value however would be pink and padparadscha (a beautiful shade of orange/pink), followed by oranges and yellows. Purple, green and colourless sapphires, have a comparatively low value, it was impossible to find auction records detailing their prices.
Sapphires generally contain very similar inclusions to rubies, however usually there are simply less of them. Because of the availability of fine sapphires, inclusions that would have little impact on the value of a ruby, would have a negative effect on the value of a sapphire. As always, it’s simply a game of supply and demand – fine quality sapphires are more readily available, therefore less deviations away from “perfection” are accepted by the market.
Common inclusions in sapphires include “silk”, needle like inclusions of rutile or hematite, and crystals of other minerals such as calcite. Because the needle like inclusions form in distinct patterns related to the crystal structure of sapphire, if enough are present a star effect is visible.
Most transparent sapphires are cut as an oval or cushion mixed cut with a step cut pavilion and brilliant cut crown. Because sapphires are more abundant than rubies, their cutting is expected to be more precise. Slight deviations that would be acceptable in ruby would have a negative impact on the value of a sapphire. If a star is present, the gemstone would nearly always be cut as a cabochon to display the star effect to its best advantage. Just on a side note of caution, if your star sapphire has a perfectly formed star, together with a flat lapped base, then that is an almost certain indication of synthetic origin – these are quite common.
Commercial-quality sapphires are commonly available in a wide range of sizes, however fine quality sapphires are rare in sizes over 5ct. Large fine quality sapphires are much more readily available than large fine quality rubies. As with all gemstones, sapphires are sold by weight and the price per carat of sapphires considered fine quality increases dramatically as the size of the stone increases
By far, the finest and most expensive sapphires sold today are from the small country of Kashmir high in the Himalayan mountains, however most sapphires sold nowadays from this location were not mined recently. Not only can stones from this location display a beautiful saturated blue colour, they are set apart and placed on a pedestal because of their soft velvety appearance. The softness in appearance of these stones is caused by minute inclusions of silk (microscopic inclusions of rutile).
Historically the most significant source, and still a major player is Sri Lanka. Most large stones found today and in centuries past, are from this beautiful - but often conflicted island, off the south coast of India.
Sapphires from the Yogo Gulch mine in Montana USA are popular for their lively blue colour, but more so because it is often entirely natural with no heat treatment necessary to bring out the vivid cornflower blue colour. In a market where consumers are becoming more aware, stones that can be sold as entirely natural certainly command a premium over their treated counterparts.
Other source of sapphire include:
Sapphires have been subject to treatment for centuries, and most sapphires on the market today are assumed to have undergone some form of heat treatment unless stated otherwise. Heat treatment can brighten or darken colour, and even dissolve some inclusions. Detection of basic heat treatment can be very difficult, especially in gemstones lacking in inclusions (the inclusions can often give indications to treatments). Even gemmological laboratories will state “no indication of heat treatment”, rather than giving a firm confirmation.
Diffusion treatment / new heat treatment
Much the same as with rubies, sapphires are heated to very high temperatures in the presence of colour causing chemicals – with blue sapphire, the chemical used is often titanium, and for oranges and yellows, beryllium. Because of the high temperatures used, the chemicals are able to penetrate into the crystal structure of the gemstone and alter its colour permanently. The colour can go all the way through a gemstone or just affect a shallow area close to the surface. Either way, the change in colour is stable and permanent.
Inherently there is nothing wrong with this treatment, it’s stable and the stones require no special care once set in jewellery. The only problem with this, and many other treatments, is disclosure of the treatment rather than the treatment itself . Gemstones are valuable to a significant extent because of their rarity (and in the case of diamonds – marketing). Although these stones may look the same as their naturally coloured counterparts, they lack their rarity and therefore should be priced accordingly. Distinguishing features of high temperature heat treatment with chemicals are blue disk like inclusions, as well as surface-related colour zoning, however often neither are present.
A synthetic gemstone is a gem that has the same chemical composition and crystal structure as its natural counterpart, the only difference being, instead of being formed in nature over hundreds of thousands of years, it was produced in in a laboratory. Sapphires have been synthesized for over 100 years. Initially these synthetics were easy to differentiate from their natural counterparts through simple magnification, but as techniques have improved, it has become more difficult to make the distinction. Most synthetic corundum is still made utilising the flame fusion (verneuil) method which was initially discovered in 1902 by the French chemist Auguste Verneuil. These gemstones show curved growth lines due to the way in which the crystals are made. Curved growth of this nature is never seen in natural corundum.
Care and Consideration for Jewellers
In natural gemstones, sapphire’s hardness is second only to that of diamond. With a hardness of 9 on the Mohs scale, it is difficult to scratch and therefore a feasible option for an engagement ring – and the choice of royalty at that! However, care must still be taken when using abrasive papers – these are generally made using a corundum grit and therefore can still scratch all coloured gemstones including ruby and sapphire (varieties of corundum). Generally, sandpapers should only be used with diamond and never with any other coloured stones. The same is true for most rubber wheels as these are also usually impregnated with some form of corundum abrasive. Pumice wheels are the only exception here, these amazing little things are a life saver for stone setters worldwide.
For rubies and sapphires that haven’t undergone treatments such as fracture filling, and do not contain cracks, no special care is needed. There are no issues putting them in the ultrasonic, they can be steam cleaned, and platting metal with them in place is not a problem. With a hardness of 9 and toughness rated as good, they should pose no major issues when setting, unless of course fractures are present.
References and Further reading:
Hughes. R, 2014, Ruby and Sapphire a Collectors Guide, Thailand, Gem and Jewellery Institute of Thailand - Any books by this guy are amazing!!!
Marco Polo, John Pinkerton. Voyages and Travels of Marco Polo
www.ruby-sapphire.com - same guy as above, his writing is witty factual and entertaining!
www.lotusgemology.com - again same guy - this guy is a gemmology god ;)