Observation environment:
JUDGE II Macbeth light booth:
- Daylight D65 (6500K)
- Incandescent light A (2850K)s
The shade:
- Main colour and/or shade determined by comparison with the GIA ‘Wheel of Color’ or Gemworld ‘World of Color’ guide.
- The colorimetric terms used will be those noted along the colour wheel or the ‘World of Color’.
Luminance or brightness:
-
Brightness characterises the perceived relative light intensity of a surface. It is the scale from white to black determined on the N1 to N9 scale in the ‘Munsell Book of Rocks Color’.
- This also corresponds to the N1 to N9 scale used to qualify the mass colour of opals at the LFG:
- Colour N1 to N4: black opal
- Colours N5 and N6: dark opal
- Colours N7 and N8: light opal
- Colour N9: white opal
- The LFG has acquired the ‘Opal Master Reference Set’, which can be used to determine and describe the ground colour as well as the colour sets for precious opals.
The colour saturation:
- This is the vividness of a colour, i.e. the intensity of colour or degrees of colour saturation for each hue.
- It is determined using the Munsell colour saturation tree.
Terminology used at the LFG :
Special cases
– ‘Padparadscha’ sapphires: an orangy pink sapphire can be described as ‘padparadscha’ if the colour is uniform in all directions. In addition, the LFG uses the colour chart drawn up by Franck Notari.
– Colour change stones: the shade must be determined in daylight (D65 at 6500K) and incandescent light (lamp A at 2850K).