Ancient civilizations were fascinated by the exceptional hardness of diamond, although colored gems were regarded as more aesthetically pleasing. Diamond was extremely rare up to the eighteenth century and was only fully appreciated after the modern type of brilliant cut, which shows it in all its glory, was developed at the beginning of the twentieth century. It is the most important gemstone today. Statistics a few years ago showed that diamonds accounted for eighty percent of the movement of money generated by gemstones. About two million carats of cut diamonds are issued on the market each year (it is the only gemstone for which reliable statistics are available), equal to a volume of little more than 110 liters.

Appearance In most cases it is almost colorless or, to be more precise, ranges from perfectly colorless (infrequent) to yellow-tinged or, sometimes, brownish. Diamonds with a definite color are extremely rare. This can be yellow, yellow-brown, or predominantly brown or, very occasionally pink to reddish, blue, blue-gray, pale green, or violet. Its luster, depending on reflection from both the inner and outer surfaces of the light incident on the table and crown, is greater than that of other gemstones, due both to its high refractive index, which facilitates total internal reflection and its exceptional hardness, enabling it to acquire a similar degree of polish.

By far the most widely used cut is the round, brilliant type,which best displays the gem's unique characteristics. But oval, marquise, pear and, more rarely, heart-shaped fancy cuts are also used. Most of these have a girdle consisting of a series of small, polished facets, while in brilliants, a girdle cut this way is uncommon and is reserved for stones treated with particular care. The special, rather elongated forms often show a dull area along the minor axis. Obviously, the better the cut, the less this band will show. The so-called emerald cut is also quite common. This has a rectangular table, stepped and chamfered. Unfortunately, this cut, which is used to reduce wastage when the stone is fashioned, is more often than not given the wrong proportions. The crown is usually too shallow (even less than 10 percent of the smaller side of the girdle) and the pavilion too deep (50-55 percent of the smaller side). The result is a stone with a lot less fire than one with a brilliant cut, or even than the rare examples of gems with correctly proportioned emerald cuts.

Diamonds are also found on the market with unusual, antique or specially designed cuts. Old mine cuts are not normally circular, but squarish, with rounded corners, or almost rectangular with rounded corners (some people call these polygonal shapes with slightly curved sides and smoothed corners “cushion” shape). The proportions of the height of the crown, the pavilion and the diameter vary a great deal in these cases, depending on the creativity of the cutter. It is still possible to find what are known as "rose" cuts, with a flat base, both in stones of some size, which are usually old or antique, and in small, shallow stones one or two millimeters in diameter, generally used in old-fashioned jewelry.

Distinctive features Hardness can be an important factor in distinguishing diamond from other stones. It is in fact the only gemstone capable of scratching corundum. The best modern imitation, cubic zirconia, is less hard than corundum, so the two can easily be distinguished by comparing them with corundum, although the results of the tests must be observed under a binocular microscope or at least a lens. Diamond's exceptional hardness is also displayed by the facet edges, which are sharper than in imitations. This is best appreciated in relation to zircon, which has brittle, easily damaged edges, and the less hard imitations such as synthetic rutile and strontium titanate. If the stones are turned between the thumb and forefinger, the two softer substitutes feel more slippery, almost oily, compared with diamond, because of this difference in the edges.

Another characteristic can be seen in the girdle, but only when it is not faceted. Due to the procedure used in turning brilliant cuts, the girdle of a diamond will have a satiny finish, similar to that of finely ground glass (in recent years a similar effect has been achieved with cubic zirconia, but no other imitation displays this). Also, when there are numerous flaws like minute cracks extending from either side of the girdle (these are known as “bearding” and are due to inexpert turning of the rough stone), it is bound to be diamond. A brilliant cut can display small facets on the girdle or extending from the girdle toward the pavilion (or more rarely, toward the crown). These are the remains of the outer surface of the uncut octahedron (nearly always with minute, crystallographically oriented shallow triangular cavities), or of the faces of octahedral cleavage. In the latter case, small steps can be observed between contiguous, specular plane surfaces. However, these details are only readily visible if magnified at least 10-20 times. Sometimes small triangular facets alone are seen extending from the girdle. Called extra facets or supplementary facets, these are produced by polishing of the facets just described or by the elimination of some small, almost superficial flaw. Although very similar to the foregoing and not usually found on imitations, these facets are less distinctive in that they could be produced on any other stone.

Other distinctive features are related to the fact that the most frequently used, brilliant and emerald cuts, are designed to make the most of the high refractive index of diamond and obtain the maximum possible total internal reflection of the light coming from the table facet. There fore if a diamond is placed with the table facet against tiny written characters, nothing will be visible through it, unless the pavilion is extremely flat. With imitation diamonds of a much lower refractive index, such as YAG (Yttrium Aluminum Garnet), something will be visible through the stone and still more will be seen through synthetic spinel and colorless sapphire. The difference is more obvious with emerald cuts than with brilliants. A similar effect, but confined to brilliant-cut stones, can be seen through the table facet. By steadily tilting a stone of lower refractive index than diamond, and looking through the table, a nonreflecting transparent triangle can be seen to appear in the pavilion, with its apex at the center and its base toward the edge of the table opposite the observer. The lower the refractive index of the stone, the smaller the angle at which this will appear. In diamonds, this effect is very difficult to see, except in poorly cut stones with very shallow pavilions. Single refraction is another characteristic that distinguishes diamond from zircon, which is strongly birefringent and from an infrequent imitation of diamond: synthetic rutile, which is even more strongly birefringent. The famous dispersion in diamond, although considerable, is much less than that of synthetic rutile and strontium titanate; but these now uncommon imitations look positively iridescent when viewed through a lens and even to some extent with the naked eye.

Given the constancy of shape and proportions, at least within certain limits, of stones with a round brilliant cut, a given weight can be said to correspond to a given diameter. lf the diameter of a stone presumed to be a diamond can be measured with some precision, one can check to see whether it has a suitable weight (in which case it will either be diamond or a stone of comparable specific gravity), or whether the weight immediately rules out the possibility of its being diamond. Generally simulants are too heavy, as in the case of cubic zirconia, GGG (Gadolinium Gallium Garnet) or, to a much less obvious extent, zircon or YAG (Fig. A.). This method could not be used to distinguish diamond either from synthetic spinel, colorless topaz, or colorless sapphire, as their relative densities are too similar, although all of them have other characteristics unlikely to deceive any but the most casual observer.

It was mentioned in discussing physical properties, that the very high refractive index of diamond is outside the range of normal refractometers and the same can be said of many of its imitations. In compensation, however, diamond has other characteristics, such as reflectivity and thermal conductivity, which are quite different (because much higher) than those of its present substitutes. Small instruments the size of a pocket calculator have been produced to measure these characteristics, making a rapid distinction possible.

Occurrence Gem quality diamonds are found in about twenty different countries, a dozen of which are in Africa, three in Asia, one in Oceania, and three in South America. By far the largest producer is South Africa, including the neighboring Lesotho, Botswana, and Namibia. Next come the Soviet Union, Angola, Zaire, and Sierra Leone. Other important areas are the Central African Republic, Tanzania, Ghana, and Venezuela, with Australia and Brazil further behind, and India now one of the last.

Value Diamond is one of the most valuable stones, together with ruby and emerald. The market value of diamonds is determined by complex grading systems that divide them into a wide range of different categories. The basic characteristics considered are weight, color, and purity. Less crucial factors are the proportions of the cut and surface finish.

The influence of weight Although a diamond weighing over 3000 carats has been found, rough diamonds of considerable weight are extremely rare. Still rarer are heavy cut diamonds, given that the average loss of weight in the cutting process is more than 60 percent. All else being equal, a diamond twice the weight of another is much rarer and therefore much more precious and will have a higher price per carat. Weight also has psychological as well as scarcity value: a diamond of 1 carat or slightly more is worth more weight for weight than one of 0.9 carats because it exceeds the conventional limit of the complete carat. The same applies at the dividing line for 2 carats, 3 carats, and so on (Fig. B). In short, the price per carat increases by stages with each complete carat number, up at least to a weight of about 10 carats, after which the effect is negligible, although such large diamonds are extremely uncommon.

The influence of color The vast majority of diamonds are colorless, or yellowish, ranging from a barely perceptible tinge to straw-yellow. Perfectly clear stones, however,are comparatively rare, but the effect of dispersion in diamonds is more pleasing on a clear ground than on a yellowish one. For these, plus psychological reasons, the more or less perfectly colorless stones are in greater demand; and value diminishes with an increase in yellow (or more rarely,brown) coloration.

ln an effort to quantify this, scales of colors of different intensities were established, with names of the color categories relating to the principal localities in which diamonds of that color were found, or to other factors. Thus the main diamond trading centers evolved roughly similar color codes matched by approximate sets of values. A very precise scale has more recently been introduced in the United States, with grades distinguished by letters of the alphabet (beginning with D, not A) and referring to data supplied by special measuring instruments rather than the naked eye. Over the last few years, European scales have been adapted to that of the United States, a series of sample diamonds being used for purposes of comparison. The four scales most widely used today and in the recent past are shown in Fig. C (4b is the same as 4a, but with the old no menclature of 1). The correspondence between scales 1 and 2 is approximate, as is their relationship to the others,whereas 4a and 4b were designed to be cross-referenced.

As a general guide, color H (white) is very good and few diamonds can boast such quality. Color I (the old “commercial white") is much better than this name would.imply, with a barely perceptible yellow tinge; and many stones on the market, particularly medium- or large—sized ones, come under categories J, K, L, or M. By contrast, “rare white” stones are much less frequent. A very rough idea of the visibility of coloration in diamonds of different grades (seen from above only, in conditions of “use,” rather than the ideal conditions according to which they are classified) can be obtained from Fig. D, which uses the classifications of the CIBJO scale. Naturally, the larger the area over which any coloration is viewed, the easier it is to see. All else being equal, the price of diamonds varies quite sharply with color. Again, as a rough guide and obviously depending on the state of the market, if a diamond with certain characteristics and weight of color H were worth 100, another with the same characteristics of color F could be worth 130, while one of color J would be worth 80, and one of color M,

The influence of purity Many diamonds contain crystalline inclusions or discontinuities (even just fractures) which reduce their transparency, by interrupting the paths of the light rays. Stones free of inclusions or visible discontinuities are described as pure, and value diminishes as visible imperfections increase.

Classification is based on visual criteria and detailed diagrams are available to establish a basis of comparison. Stones are conventionally examined under a 10x lens. The grades of the scale of purity are shown in Fig. E and are referred to by internationally accepted sets of initials, according to the English terminology. Pure stones of IF grade are uncommon; WS and VS grades are of a good level of purity, Sl and Pl grades are not so pure and P2 and P3 are the lowest categories. Naturally, stones combining perfect purity with excellent color are rare, especially those weighing several carats. If one is prepared to compromise over a single characteristic such as color, it will be easier to find stones of good purity and weight. Similarly, it will be easier to find a stone of good color and weight but mediocre purity; and those of middling color, medium purity, and small size are obviously the most common.

The above should give some idea of the complexity and also the degree of organization of criteria used for the valuation of diamonds. In addition to these fundamental criteria there are others based on the proportions of the out (when not ideal, they reduce the stone’s powers of light reflection and refraction) and surface finish, which are taken into account in establishing value. Proportion and finish are judged as "very good," “good,” "medium," or “poor." Sometimes these judgments are based on measurement, but usually they are established by the visual evaluation of experts.

Simulants and synthetics Diamond has been imitated by glass and special glass with a mirror backing to increase reflection, called “strass." It has been imitated by colorless, synthetic spinel and corundum ever since these began to be produced. Over the last few decades, moreover, it has been imitated by a whole range of synthetic and artificial products:

  • strontium titanate or fabulite, which has much higher dispersion than diamond, with striking iridescence, low hardness, considerable brittleness and density; -

  • synthetic rutile or titania, which has very high dispersion with striking iridescence, marked birefringence, high density, and low hardness;

  • YAG (Yttrium Aluminium Garnet), which has a slightly lower refractive index than diamond, a hardness of less than 9, and greater density;

  • GGG (Gadolinium Gallium Garnet), which has a fairly low hardness and double the density of diamond; '

  • Cubic zirconia (cubic zirconium oxide), djevelite or phianite, the most recent and hardest to distinguish from diamond. its density, however, is much greater, and its hardness is less than 9.

Many other artificial substances are similar to diamond, but are not used as imitations because their cost is generally much higher and their characteristics no better than those simulants already mentioned. Diamond has been and still is manufactured synthetically, in minute crystals, only suitable for industrial purposes. It has also been produced experimentally for use as a gemstone and a few gems of about 0.25-0.45 carats have been cut. But it is so hard to make the crystals grow that the cost would be much higher than that of natural diamonds obtained from known deposits.