Swiss labs harmonize ruby and sapphire colour standards

by jacquie_dealmeida | November 16, 2015 11:56 am

[1]Swiss-based Gübelin Gem Lab and Swiss Gemmological Institute (SSEF) are harmonizing their standards for the colour terms ‘pigeon blood red’ and ‘royal blue.’

While both terms have been used by the trade for centuries to describe only the finest quality rubies and sapphires, it’s not until this agreement that they will refer to stones exhibiting precise colours and quality criteria.

“By applying these harmonized standards, it is the aim of SSEF and Gübelin Gem Lab to provide the trade with unified and consistent guidelines for the use of these historically significant terms,” said SSEF director Michael Krzemnicki.

Currently, SSEF and Gübelin both have their own strict criteria for which rubies and sapphires must qualify to be called ‘pigeon blood red’ and ‘royal blue.’ Although there were some differences, the two labs found their standards were largely the same. A few minor changes were agreed upon to further harmonize them.

The following are the criteria for colour and quality:

1) Colour criteria: For a ruby or sapphire to qualify for the term ‘pigeon blood red’ or ‘royal blue,’ the colour has to be intense, saturated, and homogeneous red or blue. The exact ranges of hue, saturation, and tone are to be defined by sets of master stones.

• ‘Pigeon blood red’ is best described as a red colour, with no apparent colour modifiers (e.g. blue or brown), although a minute purplish tint is acceptable. The body colour of pigeon blood red rubies is complemented by strong fluorescence when exposed to ultraviolet light (UV). This fluorescence is caused by high chromium content combined with low iron content, resulting in the coveted distinct ‘inner glow.’
• ‘Royal blue’ is described as a saturated blue colour, either pure or with a very slight purplish tint. While ‘royal blue’ is a term historically coined for the best quality of sapphires originating from the Mogok area in Burma, sapphires from other metamorphic deposits—such as those in Madagascar and Sri Lanka—may also display the properties required to qualify for the ‘royal blue’ term.

2) Quality criteria: These colour terms can only be applied to rubies and sapphires exhibiting fine qualities and have not undergone any modification of colour and/or clarity. Any type of treatment (e.g. heating, fissure filling, etc.) disqualifies them from being described using these colour terms. Further, they must be relatively free of eye-visible or dark inclusions, and they must show a homogeneous colour distribution with vivid internal reflections.

“Clearly, such stringent sets of criteria implies that only a very small percentage of rubies and sapphires qualify,” said Daniel Nyfeler, Gübelin Gem Lab’s managing director. “This is in line with the experience and belief of both Gübelin and SSEF that historically, only exceptional rubies and sapphires were attributed these quality terms.”

Source URL: https://www.jewellerybusiness.com/news/swiss-labs-harmonize-ruby-and-sapphire-colour-standards/

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