Sapphire is one of the four recognized precious gemstones and the second hardest mineral on earth after diamond. Being the birthstone of September, this deep blue gemstone has been adored for its colour, durability and beauty since ancient times.
Sapphire Origin and Formation
The name sapphire comes from the Greek word “sappheiros” or the Latin word “saphirus” which means blue.
Like diamonds, sapphires take millions of years to form and no two natural sapphires will ever look the same. They are formed beneath the earth’s surface under immense pressure and intense heat out of the mineral called corundum (aluminium oxide) which seeps into cracks in igneous or metamorphic rocks.
Once the liquid cools it turns into colourless crystals. However, when minuscule traces of other minerals (often as little as 1%) mix with corundum they turn it into various colours such as red, pink, blue, yellow etc.
If the transition microelement is iron or titanium the corundum turns blue and we get blue sapphires. In the case of chromium, corundum turns red and gives us rubies. The other sapphire colours are caused by various mixes and proportions of iron, titanium and chromium.
Most corundum in nature is heavily included suitable only for industrial use, while gem-quality corundum is very rare. Rough sapphires come in a barrel shape that is tapered at the ends and larger in the centre. Once mined, they can be cut into any shape and used for jewellery manufacturing.
Sapphire deposits can be found in Madagascar, Sri Lanka, Myanmar, Thailand, China, Australia, Nigeria, Pakistan, Kashmir, Nepal and the United States. Depending on the location they come from, sapphires can have different chemical properties and inclusions. Gemstones from Kashmir, Burma and Sri Lanka are the most prized due to their high quality and clarity.
When corundum takes on a colour other than red, the gemstone is typically classified as sapphire, meaning there are many colour varieties in sapphires. While the “official” colour of this gemstone is blue, the other colours such as pink, purple, green, orange and yellow are sometimes referred to as “fancy sapphires”. However, this term does not apply to the colourless, black and blue specimens.
When iron present in the chemical composition, the sapphire takes on a yellow colour. When vanadium, sapphires get purple, and the best-known blue sapphires are created if iron or titanium is present in their structure.
Burmese and Kashmir sapphires usually have an intense and velvety blue colour, however, these stones are very rare nowadays. The most common sapphires today are the ones from Sri Lanka and Madagascar that come from light to dark blue colour.
In general, the intensity of blue is the most important factor with blue sapphires, meaning a big stone that has a washed-out blue colour will always cost less than a small vivid blue stone.
The most desirable sapphire colour is the intense cornflower blue that is not too dark. In general, the stones that have too dark or too light colour are not that valuable. However, light blue stones have greater brilliance than darker ones.
Another highly-priced and possibly the rarest specimen is the Padparadscha sapphire named after a delicately-coloured lotus flower. It is a unique blend of a pink and an orange sapphire resulting in a stunning peachy or salmon colour.
There are also colour-change and bi-coloured sapphires which are exceptionally rare and most people are unaware of their existence let alone know what they look like.
The ability to change colour when viewed under different types of light is a rare phenomenon in the world of gemstones. However, some sapphires display a bluish colour in daylight and reddish colour under electric lights. Bi-coloured sapphires display two different colours under the same lighting conditions.
Although both sapphires and rubies are formed in a similar mineral structure, sapphires often have better clarity. They can come transparent to opaque, transparent stones being the most valuable.
Sapphires can come with several types of inclusions such as needles (fine needles are called silk), mineral crystals, feathers, partially healed breaks that look like human fingerprints and colour zoning. Each of these inclusions has a different impact on the stone’s sparkle and light performance.
In general, inclusions make a sapphire less valuable. The price can drop significantly if the inclusions affect the stone’s durability. However, inclusions can also increase the value of some sapphires. The most valuable Kashmir sapphires contain tiny silk inclusions that give them a velvety appearance. The same silk causes phenomenal asterism in star sapphires. This effect is caused by reflections from needle-like inclusions that are oriented in specific directions, resulting in 4, 6 or 12 rays.
Sapphire Cut and Carat Weight
Sapphires come in all cutting styles, the most popular shape being oval.
It’s worth mentioning that the shape of the sapphire rough has a great influence on the shape and size of the finished stone. Since rough sapphires usually come in barrel shape the finished stones are often deep.
To achieve the best overall colour, proportions and minimal weight loss, cutters also have to focus on colour zoning (areas of different colours in a stone). For example, blue sapphires have zones of lighter and darker blue and cutters have to concentrate colour in a location that is best visible in the finished stone.
Sapphires come in size range from few points to hundreds of carats. However, the most popular sizes are less than 5 carats.
Fine-quality big stones are very expensive. For example, a 5-carat fine-quality blue sapphire can cost five times more per carat than the same quality 1 carat stone. This example is not meant to be an exact price guideline, but to illustrate how much the price-per-carat can go up as the quality and size rise.
|Chemical Name||Aluminium oxide|
|Crystal System||Hexagonal and trigonal|
|Colours||Blue, pink, purple, yellow, green, orange, colourless, grey, black and multicoloured|
|Hardness||9.00 on the Mohs scale|
|Refractive Index||1.76 - 1.77|
|Specific Gravity||3.98 ~ 4.06|
|Transparency||Transparent to opaque|
|Lustre||Vitreous to adamantine|
|Cleavage||None, but may exhibit parting|