Legend describes it as a stone of purity, honesty, trust and prosperity, bringing inner peace and protection to its wearer. Sapphire - the darling gemstone of royalty and bridal jewellery - is the birthstone for those born in September.
Sapphires are a variety of the mineral corundum – an aluminium oxide in the trigonal crystal system. These stones are most commonly sourced throughout the world from Australia, Cambodia, Myanmar (Burma), Sri Lanka, Thailand as well as Africa.
When mined in their natural rough form, sapphires can appear dark and dull until their true colour shines through once polished, cut or treated. The origins of sapphires can directly affect the clarity, carat, cut and colour of the stones and therefore determines their value and popularity to the trade. Sapphires that are mined within Sri Lanka or Thailand have been considered by many to be the most valuable due to their clarity and brilliance of colour. Like any natural stone, it is the display of visible inclusions within the crystal structure that can sometimes - but not always - indicate where in the world sapphires have been mined.
Natural corundum sapphire with crystals and feather inclusions. Image ©Gem-A Pat Daly.
Measuring nine on the Mohs scale of hardness, sapphire is well known for its good durability and refractive index of 1.76 to 1.78.
The resilience of these stones makes them highly desirable not only for the consumer trade market in fine jewellery but also to industrial businesses applying them to electronics and scientific instruments. Due to its high ability to withstand scratching, sapphires have now become one of the most popular choices for engagement rings and other jewellery pieces for everyday wear.
Colour spectrum of sapphire
Traditionally, sapphires are thought of as naturally vivid blue when in fact this gemstone can occur in a wide rainbow spectrum of colours; from canary yellow, tropical orange to fuchsia pink with the only one exception being the colour red. The red variety of corundum is otherwise known as ruby.
Sapphires get their colour from different trace elements. The presence of iron and titanium is responsible for the blue tones, while traces of chromium cause pink tones. Ruby red is a result of more chromium present in the stones chemical composition.
The Padparadscha sapphire is one of the rarer colours of sapphires, displaying a combination of pink and orange. This sapphire was named after the colour of the Sri Lankan lotus flower, both symbols of natural beauty born from something ordinary.
Left: Flux healed treated corundum (crystal inclusion + healed fractures). Right: Corundum with an iridescent fracture. Images ©Gem-A Pat Daly.
Like all gemstones, Padparadscha sapphires have their own individual colouring with some stones darker in tone and others displaying lighter mediums of pink and orange. In terms of value, this type of sapphire increases in value as the saturation of colour increases. It is the gemstones that display a medium tone of pink and orange saturation that are regarded more highly due to the classification of such gems to be pastel in colour.
Clarity and Inclusions
Several types of inclusions are found within sapphires, the most common being long thin mineral inclusions called needles. Long intersecting rutile inclusions are otherwise known as 'silk' and can create a soft milky transparency of some sapphires.
Generally speaking, the presence of inclusions within sapphire will make it less valuable if they are affecting the stone’s overall durability. However, in some cases like Kashmir crystals, the opposite is true.
Sapphires sourced in Kashmir, India often have a very slight milkiness caused by very small inclusions. Colour zoning and stress fractures scatter light to create an almost velvet optical effect without compromising the stone’s transparency.
Left: Synthetic corundum star sapphire. Right: Star ruby corundum. Images ©Gem-A Pat Daly.
The quality and appearance of stones found from certain sources worldwide can vary over time, meaning that new sources can often produce very similar stones to older established sources. This is why the clarity and presence of inclusions in a sapphire stone often suggests, not confirms its locality. The saturation of colour in star corundum vastly affects the value of the stone. Crisp and distinct visibility of a star against a vivid saturation of colour is considered to be the most valuable type of star corundum. Yet synthetic lab-made star sapphires can display a crisp star against a vivid background so it's always important to confirm with the seller if it is a natural or lab-made stone. It is these silk inclusions that can produce asterism – the star effect – in corundum. Sapphires displaying this asterism are also known as 'star sapphires' with the most famous being the star of India.
Star sapphires and star rubies can display 4,6 or 12 rays as reflections from tiny needle-like inclusions are positioned in particular directions, depending on the abundance of 'silk' or needle-like inclusions going in alternative directions.
|Localities||Common natural sapphire inclusions|
Rutile 'silk', accompanied by pinpoints of rutile. Long needles of apatite; dolomite inclusions (Mogok). Convoluted feathers, silk, hexagonal colour zoning in some stones.
|Crystal inclusions: particularly mica, pyrite, zirca, crystals, with haloes; healed fractures often resembling fingerprints; rutile 'silk', two-phase inclusions; apatite crystals. Graphite present as solids in two-or-three-phase inclusions. Elongated negative crystals and pyrite/pyrrhotite are common.|
|Australia||Strong zoning, feldspar, zircon crystals with associated haloes.|
Crystals of rutile are common.
Natural vs. Synthetic
Unbeknown to many, more than 90% of sapphires available on the market today have been heat treated. Whilst synthetic stones display an almost identical chemical composition to natural corundum, uncut synthetic specimens can often exhibit a variety of different crystal habits.
One of the key ways of identifying between synthetic and natural sapphires is by their characteristic inclusions.
The most common type of synthetic corundum is Verneuil flame-fusion. Verneuil sapphire commonly displays curved colour zones which occur when the boule forms as a rounded bottle-like form.
These curved growth zones can be detected through both a loupe and a microscope depending on the size of the stone and the visible depth of colour saturation. Elongated gas bubbles as well as induced 'fire marks' may also be seen, caused during the polishing process. These ‘fire marks’ are typical of corundum polished without sufficient care and are an indication that the stone is synthetic.
Left: Angular zoning in natural corundum. Right: Synthetic Verneuil flame-fusion sapphire corundum (curved zoning). Images ©Gem-A Pat Daly.
Additionally, whilst many blue sapphires show a spectrum of between 1, 2 or 3 bands, Verneuil blue and yellow sapphires do not show a spectrum whatsoever.
In summary, the primary difference between natural stones and those that are synthetic is their origin. Both stones share the same molecular composition with identical physical properties, hardness level and visual appearances.
What makes a natural gemstone rarer is that it created from the earth and then mined whereas its synthetic counterpart is lab-grown within a controlled environment. In terms of value, natural sapphires demand a higher price as they are more difficult to source and mine.
Flawless synthetic gemstones are easily made and can be produced at any time which therefore reduces their rarity and retail value. Whilst inclusions and flaws can be artificially created in lab-grown stones, it is natural gemstones with very few visible inclusions that are considered the most desirable.
Sapphire corundum with rutile inclusions that appear as silk. Image ©Gem-A Pat Daly.
Birthstone lore of sapphire
Historically the striking blue gemstone has been thought to symbolise honesty, trust, purity and loyalty.
Left: Natural sapphire corundum showing negative crystals + feather inclusions. Right: Synthetic sapphire corundum (Chatham) with flux feather inclusions. Images ©Gem-A Pat Daly.
The ancient belief that sapphire is an amulet of power and trust to those born in September still stands strong today. Such an idea would also explain why, in Ancient Rome and Greece, celestial blue sapphires were worn by the wealthy as a symbol of protection from the evil intentions of their enemies. The mystical lore behind gemstones is not a universal belief for many but what is indisputable is the fact that sapphire has long been a gem favoured by the rich and powerful.
Sapphire in engagement rings
Today sapphire has become the second most popular choice of gemstone after diamond for bridal jewellery and fine jewellery collections.
The most famous piece of sapphire jewellery to date remains to be the blue sapphire engagement ring, first seen in 1981 when Prince Charles proposed to Lady Diana. This beautifully decadent ring, now worn by the Duchess of Cambridge, features an 18ct oval blue sapphire with a halo of diamonds.
Top Left: Corundum sapphire with iridescent feather inclusion. Top Right: Pleochroism in corundum sapphire. Bottom: Feather inclusions in corundum sapphire. Images ©Gem-A Pat Daly.
Know someone with a birthday in September? Why not tap into the power of sapphire and give them the gift of inner peace, prosperity and joy.
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Cover image: Natural sapphire corundum with crystals and feather inclusions. ©GemA.
Featured in the Spring 2018 issue of Gems&Jewellery, Rui Galopim de Carvalho FGA DGA explores how ‘alternative facts’ have resulted in an informal nomenclature that permeates the world of gemmology. Here, he offers some examples of these long-standing quirks in terminology.
From the the Spring 2018 issue of Gems&Jewellery, here Harold Killingback FGA explores chatoyancy in sillimanite cabochons, an optical phenomenon where a band of light, known as a 'cat's eye', appears to hover above the surface of a stone, resulting in a striking lustre and colour.
Every year Gem-A gives its members and students the chance to show off their skills with the camera through the Gemstone Photographer of the Year competition. Have you got what it takes to be Gem-A's best photographer of 2018? Entries are open now and close on August 31, 2018.