color grading
1.Color system
The Grading Standard adopts Munsell color system to grade ruby colors.
2.Terminology
2.1 Munsell color system
Munsell color system is a color calibration system based on visual perception characteristics of colors from the perspective of psychology. At present, the color system is universally adopted to categorize and calibrate colors around the globe. The Munsell color system is an approach to describe colors through a combination of hue, value, and chroma in colorimetry (or colorimetric method).
2.2 Hue
The base color of hue is a color that can form equally-spaced visual intervals, including red (R), yellow (Y), green (G), blue (B), and purple (P), which constitute the base hues of ten colors when being inserted yellow-red (YR), yellow-green (YG), blue-green (BG), purple-blue (PB), and red-purple (RP). Each of the ten base hues can be further broken down to 10 equal portions, constituting 100 hues and then evenly distributed in a 360°circle. The Standard categorizes ruby colors as red (R), purple-red (pR), and orange-red (oR) for grading.
2.3 Value
Value is based on the achromatic color scale. There are an altogether of 11 value scales measured by the decimal system when ideal black with zero reflectivity is set as 0 and ideal white with 100% reflectivity as 10 and the middle and the middle is divided in equidistance. The value of the color of each hue’s highest chroma varies depending on hue.
2.4 Chroma
Chroma represents tone’s chroma. Its values go from the middle (0) and outward, increasing with the chroma of tones without an upper limit in a theoretical sense (the actual upper limit of ordinary colors is about 10 meters while that of reflective and florescent materials can reach as high as 30). As human eyes have different sensitivity to various colors, chroma does not necessarily match with each combination of tone and value.
3.Standard materials
The Grading Standard adopts the Munsell Book of Color-Glossy Collection for grading.
4.Color grading
Based on ruby’s hue, value, and chroma, its colors are divided into five grades, i.e. Red, Intense Red, Vivid Red, Pigeon’s Blood, and Deep Red, among which Pigeon’s Blood is a special grading in-between Vivid Red and Deep Red. Please refer to Table 1 below:
Table 1. Characteristics of different value & chroma grades of ruby by naked eyes
| Value & chroma grade | Characteristics observed by naked eyes | |
| Red | Red | Appears red in reflective light, medium color |
| Intense Red | Intense Red | Appears relatively intense red in reflective light |
| Vivid Red | Vivid Red | Appears vivid red in reflective light |
| Pigeon’s Blood | Pigeon’s Blood | Appears vivid and bright red like pigeon’s eyes in reflective light |
| Deep Red | Deep Red | Appears deep red in reflective light, relatively dark color |
5.Grading environment
5.1 Grading light source
A light source with a 4500K-5500K color temperature and no less than 90 in CRI should be adopted.
5.2 Grading environment
Color grading should be conducted in an indoor environment without direct sunlight. The tone of grading environment should be white or grey and a white background should be used under the grading light source.
6.Grading method
6.1 Value and chroma grading principles
6.1.1 When the to-be-graded ruby’s value and chroma are the same as a certain standard sample, then the grade of the standard sample’s value and chroma should be the grade of the to-be-graded ruby’s value and chroma.
6.1.2 Whne the to-be-graded ruby’s value and chroma are in-between two successive standard samples, then the lower grade of the standard sample’s value and chroma should be the grade of the to-be-graded ruby’s value and chroma.
6.1.3 When the to-be-graded ruby’s value and chroma are higher than the highest grade of the standard sample, then the highest grade of the standard sample’s value and chroma should be the grade of the to-be-graded ruby’s value and chroma.
6.1.4 When the to-be-graded ruby’s value and chroma are lower than the lowest grade of the standard sample, then the lowest grade of the standard sample’s value and chroma should be the grade of the to-be-graded ruby’s value and chroma.
6.2 Observation method
In the defined observation environment, gems are placed about 25cm away from the light source, the same parts of which as the standard sample are observed from the direction of worktable and compared with the Munsell Book of Color-Glossy Collection, and graded according to the color of gems against reflection color. Gems can be tilted by an angle of about 30o.