Understandably, colour is the single most important factor when evaluating coloured gems. Generally, the more attractive a gem’s color, the higher the value. Bright, rich and intense colours are generally coveted more than those that are too dark or too light. However, there are exceptions, such as the lovely padparadscha sapphire, which is valued for its delicate pastel hues.
Colour is typically described in three dimensions: hue, tone and saturation. Hues lie around a circle in the horizontal plane. Saturation lies in the horizonal plane, from zero in the center, to maximum at the outside edge. Tone varies in the vertical plane, from the lightest at the top to the darkest at the bottom.
The position of a colour on a colour wheel, i.e., red, orange, yellow, green, blue and violet. Purple is intermediate between red and violet. White and black are totally lacking in hue, and thus achromatic (‘without colour’). Brown is not a hue in itself, but covers a range of hues of low saturation (and often high tone). Classic browns fall in the yellow to orange hues.
Generally speaking, gems with hues that most closely resemble the red, green and blue (RGB) sensors in our eyes are most popular. Thus the coloured gem trinity, ruby, emerald and sapphire. But there is much about hue that is a personal preference and will depend upon an individual’s particular taste.
The richness of a colour, or the degree to which a colour varies from achromaticity (white and black are the two achromatic colours, each totally lacking in hue). When dealing with gems of the same basic hue position (i.e., rubies, which are all basically red in hue), differences in colour quality are mainly related to differences in saturation, because people tend to be more attracted to highly saturate colours. The strong red fluorescence of most rubies is an added boost to saturation, supercharging it past other gems that lack the effect.
Tone describes a colour's degree of lightness or darkness, as a function of the amount of light absorbed. White would have 0% tone and black would be 100%. At their maximum saturation, some colours are naturally darker than others. For example, a rich violet is darker than even the most highly saturated yellow, while the highest saturations of red and green tend to be of similar tone. Note that as saturation increases, so too does tone (since more light is being absorbed). However, there reaches a point where increases in tone may result in a decrease in saturation, as a colour blackens.
When judging the quality of a coloured gem, tone is an important consideration. Before buying, it’s always a good idea to consider the lighting conditions under which it will be worn. Look for stones that look good even under the low lighting conditions you find in the evening or in a restaurant, for these are typically the conditions under which fine gems are worn and viewed. Also view gems at arm’s length and look for those that are attractive even at a distance. Exceptional gems tend to look great under all lighting conditions and viewing distances.
Although specific colours are more popular than others, personal preferences are also important. The colours seen should ideally remain attractive regardless of prevailing light conditions, whether viewed indoors, outdoors, by day or by night.