Sapphire – the sumptuous September birthstone
Most people automatically associate sapphire with the blue variety but they are in fact found in the full spectrum of colour. With a long history of royal patronage, it is hardly surprising that sapphires are now some of the most desirable and valuable coloured gemstones on the planet. Unlike many coloured stones, corundum (which includes sapphire – all non red varieties – and ruby – all red specimens) are a fairly durable 9 out of 10 on the Moh’s scale of hardness. This makes them easier to work with on the jewellers’ bench because they are less prone to cleaving, cracking and chipping when being set into jewellery. Some extremely rare examples exhibit two different colours within the same stone, in a phenomenon known as pleochroism. Another highly unsual quality of a corundum is referred to as a ‘star’ effect or ‘asterism.’ Colour is the most important defining quality of a natural sapphire and blues are usually the most valuable. Colour can be subdivided into hue (red, orange, yellow, green, blue, violet and purple); tone (the stone’s reflective qualities – measured from 0, or colourless, to 10, which is black); and saturation (colour intensity as measured from 0, or grayish-brownish, to 6, which is vivid). On lab reports the abbreviation for the dominant hue is capitalised, whereas secondary hues are depicted in lower case and vivid colouration is the most desirable. Kashmir sapphires are the most highly sought after of all, due to their aesthetic brilliance and scarcity (production having ceased almost a century ago).
(People usually think blue with sapphires but all of the above are from the same species)
After cornflower and royal blue sapphires, pink examples are the next most desirable. In particular padparadscha, orangey-pink stones from Sri Lanka, are the most valuable. The clarity of a gemstone is determined by anything that impacts the transmission of light within it. All corundum are Type II stones, meaning they all contain inclusions. These can take the form of small fractures and fissures, as well as liquids, gases and foreign crystals and objects within their structure. Sometimes inclusions can actually add to the desirability of the stone, as is the case with Kasmir sapphires, whose ultra fine threads of rutile crystals, referred to as ‘silk,’ scatter the light to give them a velvet appearance. The finest corundum are said to be loupe clean, a condition in which no inclusions can be observed under inspection with the aid of loupe. These are extremely rare so care must be taken to ensure the stone is natural and untreated. The finest shapes for a sapphire are those that maximise the return of light and enhance colour, including step and brilliant cuts. Sapphires can be found in all cuts, both faceted and cabochon, and as with all gemstones their price increases exponentially with carat size, most noticably at major carat markers (1.00, 1.50, 2.00 etc). The finest examples of 1 carat Kashmir sapphires can fetch more than US$25,000 per carat and 5 carat specimens more than $50,000 per carat (these are some of the rarest coloured gemstones anywhere on earth).
(An example of a padparadscha sapphire)
A sapphire’s colour is determined by its host rock and their locations leave tell-tail variations in their appearance. Over the years gem dealers have capitalised on this by marketing stones that exhibit similar characteristics to those from famous regions as though they were of that actual provenance. After the famous Kashmir sapphire, the next most valuable hale from Myanmar (Burma). Akin to Kashmire specimens, these also display vivid saturation and are often found in the desirable royal blue colouration. The only thing setting them apart is the fact that Burmese stones lack the velvet lustre found in Kashmir examples. Ceylon (Sri Lankan) sapphires have a similar hue to Kashmires and Burmese but they tend to have a weaker saturation and lighter tone. Montanna sapphires tend to be found in paler blue-gray colours and they usually require heat treatment to really stand out. Cambodia produces violetish to greenish-blue sapphires with a a medium to dark tone, whereas Thai and Australian examples have similarities with Sri Lankan stones but do not have the same brilliance. They are usually dark in colour and are often assessed to be inky or blue-black, their dark tones maskign their colour and brilliance. Many Australian stones also show a strong green and blue pleochroism, meaning they can actually appear to be as strong blue from one side but strongly green from another angle. Fine yellow sapphires are also common in Australia and they are growing in popularity.
(The Kashmir sapphire worn by Diana, Princess of Wales and the Duchess of Cambridge)
Colour change sapphires can appear to change from a cool to a warm colour when taken from (UV) daylight to flourescent (artificial) light. This is caused by the presence of trace elemetns of vanadium. Sythetic sapphires were first created in laboratories in the nineteenth century and the techniques that have been developed make them hard to distinguish from their natural counterparts. If it is too good to be true, then it probably is! It certainly pays to look for stones that are accompanied by a verifiable lab report. Between 90-95% of corundum have been heat treated in order to reduce inclusions and enhance the stone’s colour so untreated stones come at a real premium. The results of heat treatment are usually stable and permanent so there is nothing wrong with stones that have received this process, providing it has been disclosed to the buyer. Under lab conditions it is possible to discern an unheated from a heated example, often by detecting the presence of rutile needs (which melt at a very low temperature and are hence not usually present in heated stones). In addition to heat treatments, some other enhancement techniques have also been developed over the years. Diffusion treatments are often used for blue sapphires but not usually for other colours. It is also possible to use beryllium treatments to turn lighter coloured sapphires into the more desirable padparadscha sapphires that exhibit the pinkish-orange tones. It pays to buy sapphires from a reputable dealer and inspect the certificate carefully to ensure the true provenance and characteristics of the stone.
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