Precious Stones Colour
As the name suggests natural coloured gemstones are all about colour. Colour as in diamonds is composed of hue, tone and saturation. Hue is the basic spectral colour we see. The tone is the relative lightness or darkness of a colour and ranges from white (0 percent) to black (100 percent). Generally, the lighter the tone the more brilliance a stone will have. And saturation refers to the intensity of the hue. The best range of saturation for most gems is medium to medium dark. Over-saturation usually leads to ‘over-colour’.
Beauty or ideal colour in gemstones is more closely defined by a good balance of hue, tone and saturation. And this optimum, the point at which tone and saturation will produce the most vivid hue possible – or ‘gamut limit’ – varies for each specific hue. And desirability is directly correlated with this optimal hue. Our eyesight has evolved to excel at seeing colour and humans will naturally select the most luscious hue. The brighter a colour the better, and pure hues tend to vivid.
Unfortunately, minor modifiers appearing as either ‘cool’ grey or warm’ brown modifiers are often seen in coloured gems. They become most easily discernible at darker tonalities unless present in such large amounts that a stone appears ‘dull’ from the onset. Their ‘mask’ tends to reduce brightness and colour saturation.
There are eight chromatic hues: red, orange, yellow, green, blue violet, pink and purple. All except the last two colours are ‘primary’ spectral hues which are constituents of white light. Pink and purple, on the other hand, are modified spectral hues. Accordingly, gemstones may be separated into primary colour gems, those that are of pure colour and those which possess mixed hues. Hues may further be differentiated as being either warm or cool. Observers also note that certain stones look best in the evening or during the day respectively and depending on which Kelvin colour temperature enhances the look of the stone. Consequently, the trade distinguishes between so-called ‘day’ and ‘night’ stones.
Viewing and colour grading of gemstones must always be done under consistent lighting. Accurate colour grading without visual references is challenging and risky. Only an experienced eye and continuous viewing of gems for colour comparison help as humans only have rather poor colour memory.
Gemstone colour assessment distinguishes between bodycolour and apparent colour. Bodycolour is the transmitted colour visible through the pavilion of a stone, face-down. It is the colour inherent to a material, the internal glow we perceive. The apparent colour, on the other hand, is the face-up colour expression of stone. It is the entirety of internal light refractions and internal and external reflections as well as body colour and exterior brilliance and sparkle. The apparent colour tends to be lighter in tone and is the colour used to evaluate a facetted gem. The body colour, however, gives a strong indication of the true colour potential of the rough and determines how to cut it best.
The shape, cutting style and geometry as well as proportions, symmetry and finish all influence the final colour of a cut gemstone. A well selected and executed cut optimises the potential of material and the tone as well as colour distribution. The cut ultimately creates the final face-up mosaic composed of alternating light areas of reflection and dark areas of extinction. Sometimes colour zoning affects the quality and evenness of a gem’s colour. Many rubies and sapphires are affected by inhomogeneous colour and clever cutting minimises its effect.
Colour inspection of gemstones should always be done under incandescent light. Ideally, a specimen should show the same face-up colour under varying lighting conditions. Should there be a shift of colour, poor chemistry is possibly the culprit as certain elements like vanadium may lead to subtle colour change phenomena. This is obvious in many purplish pink sapphires.
‘Multicolour effect’ which is the property of stones to show differing hues and/or different colour tones face up may also be observed. The main reason is the length light travels through a stone. It influences the colour of a stone. The multicolour effect is viewed negatively in primary-hued gems but more positively in secondary hued gems. It is not to be confused with pleochroism which is the variation in colour according to varying crystal directions in doubly refractive gems. Pleochroism is much less subtle than the multicolour effect and easy to detect.
The presence of disadvantageous trace elements may also simply lead to a ‘closing-up’ of the crystal. In such a case the crystal turns murky under different lighting. Its impact is much starker than the comparably benign effect a merely poor crystal would have on colour. Certain Burmese rubies containing excess iron are sometimes afflicted. Sadly, Iron will also compromise fluorescence in gemstones. Fluorescence has the beautiful effect of supersaturating certain colours and turning the finest reds of many Mogok rubies much more vibrant.
Yet colour is most often a reflection of the specific chemical fingerprint any gem carries.
Some gems are ‘allochromatic’, their colouration being owed to foreign colour trace elements. In others the gem’s inherent chemistry causes the colour, classifying them as ‘idiochromatic’. The chemical fingerprint of any gem can nowadays be analyzed in gemological labs using spectrometers which may help in geological source and even precise origin determination. The finest colour and quality from a pedigree source will time-proven command the highest premium in the market place. However, the beauty of a specific stone must reflect its pedigree. A mere ‘name’ or origin should never by itself command a price premium.
A gem lover’s choice of colour ultimately always remains a personal one. Regional as well as cultural preferences do affect colour perception and taste and sometimes two ideal hues coexist for the same gem species in the market. Cornflower versus royal blue in sapphires is such a case. Western countries tend to prefer more bright, slightly lighter toned ‘open’ colours than many Asian countries do, a pattern that is largely reinforced by the latitudinal effect light intensity has on the visual perception of a stone.
The final consumer also has to bear in mind gems may look darker once set in jewellery.
Gemstones can also feature special (colour) phenomena. These include moonstone sheen (adularescence), the cat’s eye effect (chatoyance), asterism or star-effect, change-of-colour (e.g. Alexandrite) and play-of-colour (opal). Fine colour-changing gems should display as dramatic an effect as possible whereas the ideal in chatoyant gems and those affected by Asterism would be the perfect colour with a modest yet ‘razor’ sharp cat’s eye or star. Unfortunately, this is rarely the case and many are heated to ‘clear’ them up of excess silk and improve colour thereby transforming them into material ideally suited for facetting.
Indeed heat as a treatment towards colour improvement has a long tradition in many gems, particularly rubies and sapphires. However, only stones subjected to low heat also referred to as ‘blown’ heat and represented on certificates by the symbol ‘h’ are widely accepted and may commonly be found in fine jewellery. Only traditional lower heat treatments have a limited impact on the crystal.
Stones subjected to elevated levels of heat and more complex and devious enhancements should always be avoided. True connoisseurs avoid treatments that enhance the colour altogether. The fundamental issue here is of where to ‘draw the line’ towards accepting enhanced stones.