Black is obstruction, meaning source of the light return is blocked by the observer. It is simulated for an observer at approximately 1 foot away looking at the stone with a single eye. Should not be visible for an ear-ring stone. You are unlikely to have someone viewing the stones nearer than a foot away from your ears.
The image is not well taken, the stones are placed too deep into the idealscope, resulting in no control leakage present at the edges and a much larger obstruction area.