Gemstone Colour Terms

Differences in reports from different gem testing laboratories, and hence efforts to harmonise the work of labs, are notorious in our industry. Indeed, conflicting results pose a challenge to a seller, and might raise doubt in the stone and the seller alike. Debates about such inconsistencies typically relate to country of origin, but also to treatment status and colours, namely trade colours such as pigeonblood red or royal blue. There is a global consensus that harmonisation would be the right thing to do, but it does not happen. The Gübelin Gem Lab does pursue harmonisation, specifically for trade colour terms. However, not all labs are equally strict in the application of agreed definitions, limiting the result of these harmonisation efforts.

Historically, the gem trade uses dedicated terms to describe specific colours and qualities of gemstones. Such terms - often reflecting regional preferences – are typically assigned to gemstones of superior quality. They tend to blend colour attributes with other quality characteristics and rarity.

Since there is no definite agreement as to precise colours and quality criteria a stone has to fulfil, there is little consistency on how these trade colour terms are applied within the industry. Nonetheless, gem labs use these colour terms on their reports to fulfil the demand from the trade.

With the use of these terms by laboratories, an international standard and harmonisation becomes increasingly pressing to foster a more consistent application and finally to protect consumer confidence. Efforts to find a shared definition and interpretation of trade terms arose already years ago within the industry to work towards the harmonisation of standards to determine “royal blue” sapphires, “cornflower blue” sapphires, “pigeonblood red” rubies or even “Padparadscha” sapphires.

 

Hamonisation of trade colour terms

With the intention to bring clarity to the industry, the Gübelin Gem Lab and SSEF compared the other laboratory’s criteria for the colour terms “pigeon blood red” and “royal blue” and found them to largely coincide. In 2015 the Gübelin Gem Lab and the Swiss laboratory SSEF agreed to harmonise their definitions and testing procedures for “pigeon blood red” and “royal blue”. However, the exact application of these agreed-upon definitions and procedures, as well as a residual ambiguity in the strictness of interpretation poses a limitation to a complete harmonisation.

Hence, harmonisation efforts can merely reduce differences. As long as lab work continues to have an element of subjectivity, lab harmonisation remains an ambition only.

As one of the members of the Laboratory Manual Harmonisation Committee (LMHC), the Gübelin Gem Lab was and keeps also participating in the harmonisation of other colour-related trade names used in our industry, including those for padparadscha sapphire and paraiba tourmaline. The Gübelin Gem Lab welcomes and supports all efforts in this direction.

The Gübelin Gem Lab’s definition of trade colour terms

The Gübelin Gem Lab is known for applying the most rigorous and stringent standards in all its work. This also applies to the use of trade colour terms. Only terms that have a global recognition and a certain general understanding of its meaning get considered. These few terms are translated into a comprehensive list of gemmological and visual criteria. For a stone to qualify for such a term, it has to fulfil all the respective criteria, enabling a consistent application of these terms. The following paragraphs describe the trade terms that the Gübelin Gem Lab applies on its reports, and the underlying criteria a stone has to fulfil to qualify.

Pigeon Blood Red

Historically, the term “pigeon blood red” was applied for rubies of the highest quality from the Mogok Gemstone Tract in Burma (Myanmar). These rubies were formed in marbles of, and show a saturated red colour, intensified by a soft red glow under natural daylight. This glow is called fluorescence, and is caused by ultraviolet light that makes part of the spectrum of daylight. Such fluorescence is only possible in rubies characterised by a very low iron concentration. With the discovery of additional ruby deposits in marbles in Burma, such as at Mong Hsu, and also in other countries, this effect and hence this term is no longer restricted to rubies from the Mogok region. Nonetheless, most rubies from places other than Burma contain higher concentrations of iron that suppress fluorescence, and consequently do not comply with the labs’ criteria. 

Colour criteria

For a ruby to qualify for the term “pigeon blood red”, the colour has to be an intense, saturated and homogeneous red. The exact ranges of hue, saturation and tone are defined by sets of masterstones.

“Pigeon blood red” is best described as a red colour, with no apparent colour modifiers (such as blue or brown). A minute purplish tint is acceptable. The body colour of pigeon blood red rubies is complemented by a strong fluorescence when exposed to ultraviolet light, as explained above. This fluorescence is caused by high chromium content combined with low iron content, and results in the distinct "inner glow" coveted by ruby connoisseurs.

Quality criteria

In terms of quality, these colour terms can only be applied to rubies that exhibit fine qualities, and have not undergone any modification of colour and/or clarity.

Any type of treatment (such as heating, fissure filling, etc.) disqualifies them from being described using this colour term. Furthermore, they must be relatively free of eye-visible or dark inclusions, and they must show a homogeneous colour distribution with vivid internal reflections.

The size of the stones is not considered a criterion, meaning that small rubies may qualify for these colour terms.

Royal Blue

Traditionally, the term royal blue was coined for high-quality blue sapphires from Burma (Myanmar), displaying a deeply saturated, blue colour, possibly complemented by a minute trace of purple.

Colour criteria

For a sapphire to qualify for the term “royal blue”, the colour has to be an intense, saturated and homogeneous blue. The exact ranges of hue, saturation and tone are defined by sets of masterstones.

“Royal blue” is best described as a saturated blue colour, either pure or with a very slight purplish tint. While “royal blue” is a term that was historically coined for the best quality of sapphires originating from the Mogok area in Burma, sapphires from other metamorphic deposits, such as those found in Madagascar and Sri Lanka, may also display the properties required to qualify for the “royal blue” term.

Quality criteria

In terms of quality, these colour terms can only be applied sapphires that exhibit fine qualities, and have not undergone any modification of colour and/or clarity.

Any type of treatment (such as heating, fissure filling, etc.) disqualifies them from being described using this colour term. Furthermore, they must be relatively free of eye-visible or dark inclusions, and they must show a homogeneous colour distribution with vivid internal reflections.

The size of the stones is not considered a criterion, meaning that small sapphires may qualify for these colour terms.

Padparadscha Sapphire

Traditionally, the term royal blue was coined for high-quality blue sapphires from Burma (Myanmar), displaying a deeply saturated, blue colour, possibly complemented by a minute trace of purple.

Colour criteria

When it comes to the trade term “padparadscha sapphire”, the Gübelin Gem Lab adopts the definition of the LMHC. A padparadscha sapphire’s colour is best described as a subtle mixture of pinkish orange to orangey pink with pastel tones and low to medium saturations when viewed in standard daylight. Any type of colour modifier other than pink or orange disqualifies the sapphire of the colour term.

Only corundum that fits within this tight and well-defined colour spectrum and which displays an even colour distribution will be defined as “padparadscha sapphire” by the Gübelin Gem Lab.

Quality criteria

Sapphires which have undergone advanced treatment techniques, i.e. involving methods such as lead-glass filling, lattice diffusion or irradiation are excluded from the colour trade term “padparadscha sapphire”. Standard heat treatment is considered acceptable. Stones that show a presence of yellow or orange epigenetic material in fissures, which affect the overall colour of the stones are not called padparadscha sapphires, even when all other conditions are met.

Cornflower Blue

The Gübelin Gem Lab applies very strict policies when it comes to the use of so-called trade colour terms. Such terms need to have a certain history, acceptance and a minimal consensus in the trade on what the colour refers to or looks like. These conditions are not given for “Cornflower Blue” nor were there common denominators identified when it comes to the perception of such colour. Therefore the Gübelin Gem Lab has decided to refrain from applying this term on our reports.