The value of a diamond is determined by 4 quality factors. These were established in Amsterdam in 1975, during the congress of the World Federation of Diamond Bourses (W.F.D.B.) and the International Diamond Manufacturers Association (I.D.M.A.). It became known as the system of the 4 C’s, after the words Carat, Colour, Cut and Clarity. The larger the diamond and the finer the colour, the higher its value is; the greater the clarity, the more beautiful and better cut, the more perfect it is.

The price of diamonds is kept stable by regulating what flows into the market. The world stock is supposedly large enough to provide a cupful of diamonds for every inhabitant of the USA.

Carat

From time immemorial the seeds of the carob tree were used to determine the weight of gemstones on the scales. These seeds have a very stable weight of 0.2 grams. In Greek this kind of seed is called Keration (in Arabic, “Kharrub”). The word carat is derived from this. Another weighing measurement used in the diamond trade is the grain, that comes from the Latin word “granum” meaning “grain”. The more or less standard weight of a grain of corn is 0.05 grams. In 1907 it was internationally established that 1 carat is equal to 0.2 grams, and equal to 4 grains. For diamonds, 1 carat is further subdivided into 100 points. The word carat is also used to indicate the gold content in gold.

• Rati — the seeds of the deadly Abrus Precatorius, were used for centuries to determine the weight of diamonds and other gemstones (1 rati = 0.2 gram).

• Kernels of corn (grains) — were also used as standard units of measure for weighing pearls and gemstones (1 grain = 0.05 gram).

• Seeds of the Carob tree — have almost the same weight as the red rati of the Abrus Precatorius and were also used to determine the weight of diamonds and other gemstones. The pod of the carob tree.

• Prior to setting the carat weight at 0.2 gram internationally in 1907, various different carat weights co-existed per region. In Bologna, 188.5 milligrams applied and in Vienna 206.1 milligrams. In 1871, traders from London, Paris and Amsterdam agreed that a carat equalled 0.205 gram. Later this became 0.2 gram. The legendary Great Mogul weighed 319 rati and 279.72 Florentine carats. In today’s carat that would have been 191.40. The diamond, which disappeared without a trace, could perhaps be the Pramaha Vichien Mani (the ‘Greatest of All Crystal’) that now sits on top of the Thai coronation crown.

• Scales

• Sieves

• Sliding callipers

• Size fan

Clarity

The crystallization of diamonds is a natural process in which other materials can be included and are often visible as black particles. Cracks or air bubbles can also be enclosed. These imperfections are called inclusions. The clarity of a diamond is determined with a loupe at 10× magnification. Most diamonds contain flaws (inclusions) which came into being during the crystallisation process. A diamond free of inclusions is called internally flawless. Inclusions which can be observed with magnifying glass capable of ten times magnification decrease the value of the diamond.

• IF (Internally Flawless)  No inclusions visible at tenfold magnification.

• SI (Small Inclusions)  Small inclusions.

• Piqué  Inclusions that can be seen with the naked eye.

Colour

The more colourless the stone, the more valuable the diamond. Only a very small number of diamonds are completely colourless; most vary in very light hues from yellow to brown. Many other distinct colours also occur, the so-called fancy colours. Colour comes about as the result of chemical substances penetrating the atomic structure: these include nitrogen (yellow), magnesium (rose), radium (green) and borium (blue). Red is the rarest colour and therefore very costly.

The colour of diamonds can be changed artificially by irradiation with electrons. The artificial colouring must be mentioned in the certificate for these diamonds: “Artificially coloured by treatment”. The colour of a diamond is determined by comparing the stone with “master stones”. These are reference stones that correspond with the colour scale for diamonds established by international professional organisations in 1978. The fancy colours fall outside this framework.

• Master stones  For colour comparison: the reference set against which a diamond’s colour is determined.

• Collier of the Maharaja of Nawanagar  The central diamond, of 135.92 carats, in this state necklace that once belonged to the Maharaja of Nawanagar, is the Queen of Holland. It was given this name in honour of the 25-year jubilee of Queen Wilhelmina in 1923. In addition, rose, green, blue and white diamonds are incorporated in the necklace. Cartier, who created the necklace, described it as “the truly superb achievement of a connoisseur’s dream” (replica).

• Star-shaped aigrette with the Hancock Red  The star-shaped aigrette that adorns the Tengolok of Sultan Hassanal Bolkiah of Brunei is set with the most expensive red diamond, the Hancock Red, and a number of yellow and pink diamonds (replica).

• Cognac on Ice, 34 carats  For a long time, brown diamonds were the least valued. The Cognac on Ice, of 34 carats, is famous despite the fact that it is brown. It was purchased in 1974 by Richard Burton, who gave it to his wife Elizabeth Taylor (replica).

• Fancy Vivid Brown, Royal Cut Diamond  0.78 ct, natural colour, VS1, Royal Cut. Set in a rose gold ring, combined with white diamonds. This stone — special both in colour and shape — was polished by Mr R. Neefs from Turnhout, Belgium and donated to the Diamond Museum in 2022.

Cut

In the past centuries, different types of cuts have come into existence, partly as a consequence of technological progress. Before 1400 the given crystal shape was used, such as the octahedron — two pyramids with their bases together. Later, the better-reflecting table-shaped cut came into fashion. At the end of the 15th century, the Antwerp polisher Lodewijk van Berken invented the revolutionary polishing scaife. As a result of this it became possible to cut symmetrical facets. Thus, in the 16th century the rose cut, with a limited number of facets, came into vogue in Europe. In the 17th century, diamonds were for the first time cut in brilliant shape with its many facets. Depending on its proportions, the round brilliant with 57 facets is the ideal cut.

• Diamond projector  The proportion meter, also called a diamond projector, is an instrument to control the exact proportions of diamonds in brilliant cut. Various screens are used for different sizes and different standards, the American and the European standard.

• Cut shapes: Emerald, Princess, Brilliant, Triangle, Marquise  Specimens of well-known diamond cuts.