BlogTrue Colours with Tustains' Resident Valuer

True Colours with Tustains' Resident Valuer

True Colours with Tustains' Resident Valuer


I am lucky; I get to do what I love as my job. When I was seven years old, one of my Dad’s customers gave him a box full of minerals. I was spellbound and I fell in love with gemstones. Now, as the valuer at Tustains Jewellers, I get to see fantastic jewellery every day and there is always something different coming through the door. The most common valuation is a diamond ring, but as a gemmologist, I am always interested to see the coloured stones which come into us, be it for repair or appraisal.

Coloured stones pose different challenges to a valuer than diamonds. They are first identified using standard gemmological equipment and are then graded for quality of clarity, colour and cut, but diamonds tend to be ‘clean’ and free from inclusions whereas coloured stones typically contain many more, so they cannot be graded in the same way. I base my coloured stone assessments on the Gemmological Institute of America’s coloured stone grading system which is internationally recognised, but before the quality is graded the stone must be identified.

One of the most important aspects of appraising coloured stones is to determine whether they are natural or manmade. For example, a manmade or synthetic ruby is exactly the same as its natural counterpart but produced in a lab where the colour and clarity can be controlled producing a very high quality stone at low cost. To tell the difference, I look at the stone under a specialist gemmological microscope in order to check the inclusions and structure of the stone. Inclusions occur as the gem forms, so they are crucial clues. If I were to find a crystal of another mineral inside the stone, it is most definitely natural whereas if I were to see gas bubbles this is diagnostic of a manmade stone.

Many coloured stones are subjected to treatments to improve their colour or clarity. A natural untreated sapphire for instance will be much more expensive to replace than one which has been heated or manmade. Treatment is such common place that unless there is proof that a gem has not been treated, one must assume it has been. Sometimes stones will be certificated by a gemmological lab as being ‘untreated’.

In such a situation, the inclusions are again indispensable for the gemmologist, because they themselves are changed by the heat treatment. Natural stones contain small crystals of other minerals that were present when they formed. The process of heating will affect these crystals and alter their appearance.

A common example would be the mineral rutile inside sapphires; this occurs as very fine needles, which has led it to be called ‘silk’ in the trade. In a treated stone the silk needles can break and appear as dashes of small needles through the stone. When they are seen to be unaltered it shows the stone has not been treated.  This amazing purple sapphire is certified to have natural, untreated colour and the stunning silk seen in it is key proof. The silk is so fine that it is not seen to the naked eye, but when viewed under special lighting and magnification the sapphire’s hidden internal beauty and identity is shown.

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