A black opal is more valuable than its lighter counterparts, but how do we define "black" opal? An opal consists of two defining pigments - the 'play-of-colour', which is the vivid shine near the opal's surface:
and the 'body tone', which is the colour of the nobby underneath the play-of-colour.
It is the shade of the body tone that determines if an opal is black or not.
Body tone is generally represented through a scale, N-1 being the darkest and N-9 being the lightest.
Black opal is generally regarded to be between N1 to N3.