Step 4: Gem Grading
Appraising Opals: Terminology
Central to the GAA nomenclature is the body tone description. Body tone is the base color of the stone, without taking the play of color into account. All opal tones, from black to white, receive a value from N1 to N9, rather than the 0 to 10 values for colored gems.
Under this system, any opal with a body tone of N4 or darker can properly be called a black opal. Those gems with a tone of N5 or N6 are termed dark opals. N7 and N8 are termed light opal, while N9 is reserved for white opals.
Note that some opal body colors have a distinct hue, not just something measured on a gray scale. These are still graded according to the standard above. However, you should add the hue to the description. For example, an opal with an orange body color and N6 in tone would be described as a dark orange opal. An opal N8 in tone…