Formation

Sapphires form under very specific geological conditions deep within the Earth's crust. Composed of aluminium and oxygen, they can only crystallise where silicon, one of the Earth's most abundant elements, is scarce. This process takes place around 10 to 15 miles beneath the surface under immense heat and pressure.

Over millions of years, sapphires formed in two main environments. Some developed within metamorphic rocks, such as marble and gneiss, as continents collided and mountain ranges like the Himalayas were pushed upwards. Others crystallised deep within the Earth before being carried to the surface by volcanic activity. Sapphires from volcanic deposits, including many from Thailand, Cambodia and Australia, are often darker in colour.

Sapphire Mining and Rarity

Today, much of the world's sapphire production comes from river gravel deposits, where crystals have been washed out of their host rock by erosion over millions of years. In Sri Lanka, these gem-bearing gravels have been mined for more than 2,000 years, and the city of Ratnapura is appropriately known as the "City of Gems". While sapphires are found in many parts of the world, deposits capable of producing stones of exceptional colour and quality remain comparatively rare.

Origin

Although corundum occurs all over the world, gem-quality sapphire is far rarer, and where a stone was mined plays a major role in its value. Origin can be difficult to trace, but the inclusions within a sapphire can offer valuable clues about where it was formed.

The most coveted sapphires come from Kashmir, where microscopic inclusions create the gemstone's distinctive velvety blue appearance. After Kashmir, the most sought-after origins are Burma (Myanmar) and Sri Lanka (formerly Ceylon). Burmese sapphires are prized for their rich, royal blue colour and their inclusions scatter light, creating a silky appearance often likened to that of Kashmir. Sri Lankan sapphires are traditionally known for their exceptional clarity and are an important source of natural fancy (gemmological term for all non-blue varieties) coloured sapphires. Other sources around the world produce sapphires with their own distinct characteristics, from the inky blues of Australia to the delicate pastel tones of Montana, USA.

The Discovery of Kashmir Sapphires

The world's most celebrated sapphires were discovered by chance. In 1881, a landslide in the remote Himalayas exposed a deposit of blue crystal in Kashmir. Initially traded for salt, the stones' true significance was not immediately recognised. Situated at an altitude of around 4,500 metres in an isolated and inhospitable region, the mine was notoriously difficult to access and could only be worked for a few months each summer when weather conditions allowed. By 1887, the deposit was largely exhausted. With production limited to a brief period and no significant new finds since, Kashmir sapphires are exceptionally rare, helping to sustain their legendary status. 

Grading and Determining Value

Sapphires are not graded in the same way as diamonds, although many of the same factors are considered, including colour, clarity, cut and carat weight. For sapphires, however, colour and carat weight are generally given greater emphasis, while clarity plays a less significant role in determining value. Unlike diamonds, a sapphire's geographic origin (such as Kashmir, Burma, or Sri Lanka) and any treatments it has undergone, particularly heat treatment, are also key factors in assessing its overall market value.

Colour

Colour has the greatest influence on a sapphire’s value. In general, the more intense and consistent the colour, the more valuable the stone. While colour grading systems have been developed for sapphires, there is no universally adopted standard comparable to the D-to-Z scale used for diamonds. Instead, the GIA grades colour on three characteristics:

• Hue: The colour itself. The most highly valued sapphires range from deep royal blue to a lighter cornflower blue, though the perfect shade for you is arguably a matter of personal taste.
• Tone: How light or dark the stone is. The finest sapphires strike a perfect balance, neither too pale nor too dark so they appear almost black.
• Saturation: How intense and vivid the colour is. Vivid sapphires command higher prices, while greyish stones are worth less.

Although best known as a blue gemstone, sapphires occur in almost every hue except red, which is classified as ruby. This remarkable range of colours is one of sapphire's defining characteristics. These fancy sapphires range from pink, yellow and green to the highly prized padparadscha, a captivating blend of pink and orange named after the lotus flower and often likened to a sunset. Some sapphires even change colour depending on the light source. Known as corundum's chameleons, colour-changing sapphires typically appear blue in daylight before shifting to violet or purple under incandescent light.

Clarity

Most sapphires contain some inclusions, and their value depends on how visible they are. The most common are fine needles of the mineral rutile, known as silk, along with tiny crystals, fingerprint-like healed fractures and colour banding. 

Some inclusions can even enhance the sapphire. Silk, if deep enough within the stone, can actually improve a sapphire’s overall appearance. By scattering light across the stone, it gives the colour a softer, more even glow. When silk needles cross, they can also create a star effect, known as asterism, when the stone is cut with a curved upper surface, called a cabochon cut.

Cut

A sapphire's cut is essential to showcasing its beauty. While diamond cutters aim to maximise internal refraction and sparkle, sapphire cutters focus on achieving the richest colour possible. Rough sapphire typically forms as a barrel-shaped crystal, so cutters study its colour zoning and tone to find the best orientation while keeping as much carat weight as possible. As a result, oval and cushion cuts are the most common shapes. Stones with more noticeable inclusions or uneven colour distribution are often fashioned as cabochons, either to highlight the star effect or to disguise undesirable colour banding.

Carat and Size

Like diamonds, sapphires are measured in carats. One metric carat is equal to 0.2 grams. Blue sapphires can measure from just a few points to hundreds of carats, although most commercial-quality blue sapphires weigh under 5.00 carats, and prices rise sharply at key weights such as full or half carat increments. In 2014, the 392.52ct Blue Belle of Asia sold for $17.3 million, a world record for a sapphire. However, the weight alone does not determine a sapphire’s value and must be judged alongside colour, clarity and cut.

Treatment

Coloured stones have been enhanced for centuries, and most sapphires sold today have been treated to improve their colour and clarity. Most stones are heat-treated in specialised furnaces, a treatment that is permanent and stable. Whatever the stone has undergone, its treatment should always be disclosed, and untreated sapphires of fine colour are coveted and much more valuable.

Synthetics and Imitations

When buying a gemstone, it is important to know exactly what you are looking at. Some gemstones simply resemble a sapphire and can be used as imitations. However, synthetic sapphires are quite different. Created in a laboratory, they share the same chemical composition, crystal structure and physical properties as natural sapphires, and are therefore considered ‘real sapphires’ created in an artificial environment. Usually clearer than natural stones, they offer an opportunity to buy a sapphire at a fraction of the price. However, natural sapphires continue to command the greatest interest among collectors.

Certification

It is more important than ever to have gemstones independently assessed, and gemmological laboratories have become vital players in the gemstone market. The principal laboratories to consider are SSEF (Swiss Gemmological Institute) and Gübelin in Switzerland; alongside AGL (American Gemological Laboratories) in the United States; and GCS (Gemmological Certification Services) in London.

A laboratory report can verify a sapphire's origin, identify any treatments it has undergone, and provide an objective assessment of its colour. Descriptions such as ‘Vivid Royal Blue' or ‘Cornflower Blue' can significantly influence a stone's value, making a laboratory report an essential part of buying, selling and collecting fine sapphires.

What to Look for When Buying a Sapphire

• Prioritise colour over size
• Establish whether the stone has been treated
• Consider the origin (if documented)
• Request a laboratory report for significant purchases
• View the stone under different lighting conditions

Dreweatts Auction

With three Fine Jewellery auctions each year, Dreweatts offers the finest jewels from the world-famous ateliers of Cartier, Boucheron, Van Cleef & Arpels, among many others, antique and period pieces and jewels set with diamonds, important coloured stones, such as rubies, sapphires and emeralds, as well as natural pearls.

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