A Black Opal is a natural solid stone with a black or nearly black body tone (i.e. background appearance).
Whether an Opal is a black or not can be determined by merely closing ones eyes and squinting at the face of the stone. All colours are ignored and the overall body tone (blackness level) can then be seen. When this is compared to the scale of blackness shown below it is possible to identify how black the stone is.
Only stones achieving values N1 to N4 on this scale are considered Black Opals – attracting the additional value associated of this class of Opal.