Gemstone identification

Posted on February 04 2019

Gemstone identification

Gemstone identification

Gemstones of all types have long played a central role in historic tales of romance, conquest and adventure. There’s the Black Prince’s Ruby – a 170-carat gem and one of the oldest Crown Jewels, it has been owned by the great rulers of England all the way back to the 14th century and is one of the biggest uncut spinels in the world. It sat proudly at the centre front of the crown of Queen Victoria at her coronation in 1838, nestled among the other 3000 gems that made up the crown.

Yet one very important aspect that the history books don’t tell us about is the gemstone identification process. Many of us might assume that telling the difference between a genuine diamond and a synthetic one would be easy, but that is not always the case as the fake ones can often be very convincing. With so many imitation gems in circulation, gemstone identification is a valuable talent.

Keep it in the family

All gemstones belong to different families and within each family, the stones share a number of common properties such as crystal structure or chemical composition. The main families, and those which include highly sought after stones, are: Corundum (including Ruby, Sapphire); Quartz (Onyx, Amethyst, Rose Quartz, Tiger´s Eye, Cat´s Eye Quartz, Citrine); Beryl (including Emerald, Aquamarine), and Feldspar (including Moonstone and Amazonite).
Identification begins with an assessment of the structure of the uncut gemstone in order to classify it into the right family. Diamonds, for instance, have an octahedral structure (imagine two four-sided pyramids glued at the base), while quartz usually come as six-sided prism ending on a six-sided pyramid.

But how can you distinguish the real gemstones from the fake?

Faking it

Any stone that feels rough or sandy will probably not be a gemstone. Similarly, if a stone is easily malleable, it is unlikely to be the genuine article as real gemstones can only be shaped using professional tools due to their specific crystalline structure. Gemstone identification charts from official institutes can also be used to assist the process; these are usually highly detailed and extremely helpful.

There are a number of telltale signs that will reveal if your stone has been lab-made (synthetic) as opposed to naturally formed. Inclusions shaped by fingerprints such as nail shapes or V-shape patterns mean that your stone is synthetic. Synthetic stones have curved rather than angular growth patterns inside and, for a cruder test, platinum or gold platelets will stick to synthetic types.

Imitation gemstones are also, unfortunately, all too common especially ruby, emerald and sapphire for which the imitation market have been using highly sophisticated techniques. With imitations, the surface of the stone will appear uneven and feature flow lines, which appear as swirl marks. The stone may feel lighter than the genuine article.

Assembled stones are another form of fake gemstone, by which natural gems are mixed with synthetic materials. With this type, there may be differences in the colour and luster.

High intensity

When the gem is held between tweezers and scrutinized in natural daylight (or under a special gemmologist´s lamp), it is possible to assess its colour, transparency and weight, as well as its specific hue, tone and strength. The National Gemmological Institute publishes a fascinating guide where you can select the hue that most closely matches your stone. When it comes to assessing the tone, the Gemmological Institute of America uses seven tone levels to cover every coloured stone. When you consider how a classic colourless diamond so closely resembles a cubic zirconia, this just enhances how important it is to make a close assessment.

The saturation refers to the intensity of the gem’s colour based on whether the hues are muted or more vibrant. Gems in cooler shades such as blue or purple can be analysed by their level of greyness – the less greyness, the stronger the saturation. With stones of warmer, sunny colours such as yellow or orange, the less there is within the colour, the better the saturation.

Cutting edge

Some gemstones can also be identified by the way they have been cut. The faceted style is seen frequently, where the gem features several flat, carefully arranged surfaces. This is a style designed to bring out the stone´s brilliance and is usually seen with diamonds. Manja uses the faceted style for the majority of our gemstones, in order to reflect their light and sparkle.

The stone’s transparency and gravity is then considered; it will be determined whether it is transparent, translucent, or opaque and the level of fire, if any, will be assessed. Fire refers to the visible dispersion of white light across a stone and is classified by strength.

Tossing it carefully between the palms and comparing its weight to its size can assess the stone´s gravity. This is a vital step as the difference in density between stones means that, for example, a one-carat sapphire will appear smaller than a one-carat emerald.

Natural phenomenon

Now that the story behind your gemstone is beginning to unfold, it is time to identify any phenomena. Using a sharp light beam, it is possible to examine the stone’s surface and check for special characteristics such as adularescence (floating shimmer), aventurescence (sparkle) or cat´s eye effect. All of these phenomena are rather rare and many stones have none but checking if they are present will always be part of the identification process.

Cat´s eye effect is also known as chatoyancy and refers to how the light reflects as a band within the stone, most often seen with the tiger´s eye quartz, tourmaline and moonstone. Other phenomena include the starred effect, which occurs mainly with rubies and sapphires. This describes how a polished sapphire stone reflects a blue-green, hazy image of a star with six points, caused by light reflecting from the stone´s inclusions. The Black Star of Queensland is one of the most famous starred sapphires in the world; after being displayed in museums for many years, this 733-carat stone is now privately owned by those who can proudly describe it as the largest gem-quality starred sapphire in the world today.

Alexandrite is a stone that exhibits an extremely rare, colour changing phenomenon, whereby it shows as green/blue-green in colour in the daylight but displays soft hues of red and purple when viewed under bright light. This quality makes this already rare stone even more special; despite its relatively recent discovery in 1834, alexandrite boasts a fine backstory as it is widely believed to have been named for Russian tsar Alexander II.

Out of the darkness

The gem can then be viewed with the naked eye and a 10x loupe to ascertain its lustre. A close-up assessment will also work out if there are any marks on the stone and how the surface of this area has been affected – questions such as whether the chips are uneven or more regular will be considered, as well as the lustre of the chipped space.

A refractometer will then be used to work out the stone’s refractive index. Just a small dot of Refractometer Index fluid will act as a link between the stone and the crystal hemicylinder, with the largest or highest polished surface of the stone used for the reading. Rough or unpolished stones will not offer a reading. An initial reading is taken without magnification, and a more specific reading is then taken using the magnifier. The best readings are taken by moving the head slowly and continuously to ensure catching the best glimpse of the reflection.

As the end of the process draws near, birefringence testing will determine the stone’s identity. While birefringence may seem an alien concept to many of us, it´s actually something that can be easily understood. Birefringence refers to the difference between a gemstone´s highest and lowest refractive indexes, i.e. how light is spread throughout the stone and how strong or full the colour appears to the naked eye. The larger the difference, the more striking the effects of double refraction will be.

This can be seen with the amethyst, which may appear very dark when examined from one side but have a far stronger, fuller colour from the other. This analysis helps stonecutters decide how best to cut and shape a stone to show off its best qualities.

Colour chart

Birefringence can be used to describe not only lighter and darker shades of the same colour but also stones that seem to display different colours from different angles. Thanks to its vibrant hues, the sapphire provides a wonderful example of this. It may appear blue from one side and green from another, or even yellow and blue for those really unique examples.

Birefringence testing is completed using a refractometer and turning the stone by around 30 degrees in a series of six turns. The gem must also be placed face down on the lower lens and looked at through the top lens, turning the analyser a full 360 degrees to see it in all lights. Stones that have a higher RI will be laid on their sides to avoid light beams interfering with the results.

Assessing the stone´s refractive index (and whether it is singly or doubly refractive) is a process that can only be used for transparent and translucent stones. Other stones such as a diamond or peridot have a uniform colour.

This process is one that has helped to identify gems throughout history. The Black Prince’s Ruby that is set on the Imperial State Crown is actually a 170-carat red spinel – all red gemstones were referred to as ‘rubies’ until very recent times and these two gemstones have an extremely similar molecular mix, but they can be identified by their optical qualities: a ruby is dichroic (meaning two colours – birefringent) and a spinel is singly refractive.

For every wearer, each piece of jewellery represents something special which is why it´s important to make sure all of the pieces you own are valuable not only for their sentimental aspect but for their quality. The gemstone identification process is so intricate and refined, enabling us to produce the finest quality natural gemstones and striking pieces of jewellery.

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