The 4Cs: how to read the value of a diamond

From a distance the value of a diamond looks like a riddle, yet it is clearly regulated. Four criteria, known internationally as the 4Cs, describe it completely. Whoever reads them understands why two stones of the same size can lie far apart in price.

The cut is the only one of the four criteria determined by people, not by nature. It decides how much light the stone returns. An excellent cut makes a diamond look livelier than a larger but poorly cut stone. For radiance it is the most important C.

For white diamonds the rule is: the less colour, the rarer and more valuable. The international scale runs from D (finest white, colourless) to Z (clearly tinted). Fancy colour diamonds in champagne, yellow or blue follow their own logic; there colour is not a flaw but the value.

Almost every diamond carries tiny inclusions, traces of its growth. The clarity grade describes how visible they are. The scale runs from flawless to clearly included:

• FL / IF: flawless, no inclusions under tenfold magnification.
• VVS1 / VVS2: very, very small inclusions, not recognisable to a layperson.
• VS1 / VS2: very small inclusions, hard to find under the loupe.
• SI1 / SI2: small inclusions, recognisable with the loupe.
• I1 to I3: inclusions, some visible to the naked eye.

Carat measures the weight: one carat equals 0.2 grams. Because large rough stones are rare, the price per carat rises disproportionately with the weight. A stone of one carat costs considerably more than two stones of half a carat each.

With a classically set ring the carat weight often dominates perception. In the glass this shifts. Because the glass body acts like a lens and gathers the light, it is above all the cut that decides the effect. A precisely cut brilliant gains presence in the glass without needing more weight.