Colour Terms: Pigeon Blood Red and Royal Blue
Background
Pigeon blood red and royal blue are trade color terms that lack scientific (colorimetric) definitions and, to date, no international standard has been established for them. Despite this, there is a strong demand within the trade for an independent assessment of color and for the inclusion of color terms on laboratory reports. Historically, these colors have been perceived as very rare and exceptional in quality, significantly impacting their value in the market.
SSEF uses the terms pigeon blood red and royal blue only for rubies and sapphires:
- which exhibit strong red or blue colour saturation
- which are untreated
- which meet specific chemical and spectroscopic characteristics
- of fine quality in terms of inclusions, transparency, and colour homogeneity
Ultimately, they are colour terms and thus assessed using mastersets of rubies and sapphires according to an internal laboratory protocol.
Pigeon Blood Red
The fundamental criterion for pigeon blood red is colour. The colour of a ruby is graded by comparison with a masterset of (natural) rubies. The main parameters hue, saturation and brightness are observed. Only rubies with a strong saturation and a “vivid” red colour (similar to red traffic light) with no or only slight purplish colour tint are accepted.
The body colour of pigeon blood red rubies is complemented by a strong fluorescence when exposed to ultraviolet light (LWUV: strong, SWUV: medium – medium weak).
The fluorescence is caused by high chromium content combined with low iron content. It results in a distinct "inner glow" when exposed to sunlight (which contains UV), an effect which has been coveted in the trade historically. Rubies with very high chromium content and low iron concentration however show reduced fluorescence and do not qualify.
Pigeon blood red only applies to rubies with no to insignificant zoning when viewed from the top.
Rubies may have fissures or cavities filled with an orange substance like iron hydroxide or orange oil. Even when localized, this can significantly alter the stone's colour. Therefore, the term "pigeon blood red" is not applicable to such stones.
Proportions and the cut of the stone (e.g. presence of “window”) are also taken into account as criteria for pigeon blood red.
The stone must be relatively free of eye-visible or dark inclusions when viewed from the top with approx. 20 cm distance to the ruby. Inclusions and weak to medium “silk” in zones which do not (or only slightly) affect perceived colour are acceptable.
The ruby must show vivid internal reflections.
Royal Blue
The fundamental criterion is colour. The colour of a sapphire is graded by comparison with a masterset of (natural) sapphires. The main parameters hue, saturation and brightness are observed. Only sapphires with a strong saturation and a “vivid” blue colour but no additional colour tint (greenish, greyish or purple) are accepted.
Royal blue corresponds to a blue of strong saturation, either pure or with a very slight purplish tint. The term was historically coined for the best quality of sapphires originating from the Mogok area in Burma, it is also applied for sapphires from other metamorphic deposits, such as in Madagascar, Sri Lanka, and Kashmir.
The UV-Vis absorption spectrum of the stone must be dominated by Fe2+ - Ti4+ Intervalence charge transfer features.
- Homogeneity of colour (orientation of cut, pleochroism)
- Good proportions, avoiding dominant “window situation”
- No apparent colour zoning
- No colour modifier by fissure fillings
- No apparent and disturbing inclusions
- Good transparency, distinct internal reflections
General Comment on Colour Observation
Colour observation is based on three factors:
1) Light source (emission characteristics)
2) Observer (protocol and training)
3) Observed item (e.g. ruby or sapphire)
To grade colour consistently, the first two factors have to be defined and standardised as much as possible. This is why SSEF has developed a dedicated Light Box for colour grading.
Furthermore, colour observation takes place from the top and a ruby/sapphire are tilted 20° in all directions by the observer.
