Opal Body Tone
The body tone of an opal (also termed base tone or background colour) refers to the relative darkness of the background which is contrasting with the opal’s play-of-colour. It is assessed face-up while ignoring the play-of-colour and may range from N1 (jetblack) to N9 (transparent)
Black Opals show a play-of-colour within or on a black body tone compliant with N1 to N4, when viewed face-up.
Dark Opals show a play-of-colour within or on a dark body tone compliant with N5 to N6, when viewed face-up (these are also known as Semi-black Opals).
Light Opals show a play-of-colour within or on a light body tone compliant with N7 to N9, when viewed face-up.
Vividness of colours is of paramount importance with the brightness of an opal being directly related to price. As several levels of brightness can not be observed in the face, it is the overall brightness which used to classify it as Subtle (b4-b7), bright (b2-b3) or brilliant (B1).