The colour yellow in diamonds is most desirable when highly saturated. In what are referred to as "Cape" stones, the yellow is only slightly detectable and can, hence, make the diamond appear less bright. The colour in such stones is caused by the substitution of carbon atoms in the diamonds with groups of nitrogen atoms.
Less than one percent of the world's known diamonds are classified in the fancy range of yellows, defined by the fact that the yellow hue they possess is apparent to the eye and considered to be attractive. Rather than possessing aggregates of nitrogen atoms, their colour is caused by the actual replacement of carbon atoms with isolated nitrogen ones.
In the yellows, the vivid range is the most valuable and fine examples are sublime. Sometimes, they can appear slightly orangish, but not in the case of the present stone. It is of a yellow as saturated and pronounced as that which one sees in springtime daffodils, hence why they are sometimes referred to as "jonquilles".
It is of interest to clarify the grading system of fancy vivid, fancy deep and fancy dark which can sometimes be confusing to the non-specialist. All is relative to the tone and saturation of the colour, tone being defined as the variation in shade of a colour, and saturation, as the intensity of the colour. A yellow diamond of light to mid-tone and strong saturation would fall into the vivid range. Another example of darker tone and strong saturation would be graded fancy deep yellow. When diamonds in the yellow to orange hue range are dark in tone and weak to moderate in saturation, the colour description would always include a component of brown. Diamonds with these attributes would fall into the fancy dark category.