The factor of colour is given an importance unequalled in the evaluation of any other gem. By far, most diamond gems appear colourless to the untrained eye and only a few gems display decided hues such as one expects to see in varieties of corundum, for example; these are ‘fancy’ diamonds, very rare and very costly. The apparently colourless gems are painstakingly subdivided into numerous if somewhat artificial, colour grades, the nuances between which are seldom apparent when gems are mounted in jewelry. Nevertheless, some form of colour grading is employed by every dealer in diamonds and affects directly and substantially the price that is charged.
The grading of diamonds for colour is the art of classifying them on a scale from colourless to obviously yellow. This statement is based on the fact that most diamonds encountered in the trade range from absolutely colourless to some tinge of yellow or faintly 求婚戒指. Those that are decidedly yellow or coffee in hue are fancies and sold as such.
The overly free use of the term ‘blue-white’ is much less evident now than several decades ago when jewelers used this term more freely in their advertisements and were rightly regarded with suspicion because of the very real rarity of diamonds, which actually display some tinge of blue. A survey by the Gemological Institute of America some years ago showed that only one diamond out of 500 examined under scientifically adjusted lighting conditions actually qualified for the term ‘blue-white’. Needless to say, diamonds of this hue command a very high price. Even higher prices are asked for fancies, particularly those, which are blue to rich blue, red, pink or green, with lesser prices asked for golden, yellow and rich browns, greens and blues.
The factor of clarity refers to the absence of inclusions or flaws. The term ‘perfect’ is now seldom used; instead, those diamonds which do not reveal inclusions or flaws when examined by an expert eye under 10-power magnification rate the term ‘flawless.’
The final factor of cut refers to the mechanical shaping and polishing of a diamond crystal into a faceted gem, giving due regard to proper proportions, symmetrical outlining and placement of facets, accurate meets or junctions between adjoining facets, proper girdling and culeting and surface finish upon facets. Diamonds too deeply or shallowly cut are less brilliant than they could be, sometimes seriously so. Excessively thin crowns (‘swindled’ crowns) cut down dispersion substantially, although brilliance can be quite satisfactory. In general, overlapping facet junctions, extra facets, or uneven facets do not seriously detract from brilliance and dispersion, providing that the basic proportions are nearly correct; however, careless work is usually a sign of poor material and should also be cause for rejection of any stone in which it is glaringly evident. Careless polishing, usually shown by scores or ‘wheel marks’ on facet surfaces, results in some loss in brilliance and dispersion.