Cut, colour, clarity, carat, in that order
The four C are taught in the order that matters and bought in the reverse. Getting the order right is worth about thirty per cent of the price.
At a glance
- The only human variable
- Cut, and it decides how the stone behaves in a room
- Colour, sensible floor
- G or H in white gold or platinum, I or J in yellow
- Clarity, sensible floor
- VS2, or SI1 if the inclusion is not under the table
- Carat, the trap
- Weight rises with the cube, the face you see with the square
Two stones, same price. One is 1.20 carats, colour H, clarity SI1, cut graded excellent. The other is 1.50 carats, colour H, clarity SI1, cut graded good. In the case they look similar. In a restaurant with four candles and a downlight, the first one is alive and the second is a piece of glass. The difference is entirely in the grade most people treat as a formality.
Cut, which is not shape
Shape is round, oval, emerald, pear. Cut is how well the facets are proportioned and finished, and it is the only one of the four that a person chose rather than the ground provided. A round brilliant has 57 or 58 facets whose angles decide whether light entering the crown reflects back out of the crown, which is brilliance, or leaks out of the pavilion, which is a dull stone.
Three numbers carry most of it. Table percentage between 54 and 58. Depth percentage between 59 and 62.5. Crown and pavilion angles around 34.5 and 40.8 degrees. A stone cut to those proportions returns light. A stone cut deep to hold weight, which is how a cutter maximises the carat number out of a given piece of rough, holds the extra weight in the pavilion where you cannot see it and pays for it in dead light.
A deep cut stone is heavier, more expensive and smaller to look at. It is the only way to be worse off in all three directions at once.
Colour, and where it stops mattering
The scale runs D to Z, with D as colourless. Nobody can see the difference between D and G in a mounted stone without a comparison set, and almost nobody can see the difference between G and I unless the setting is white metal. The colour of the metal does the work: yellow gold makes a J or K stone read perfectly white by contrast, platinum makes anything below I look faintly warm.
Which is why the honest recommendation is conditional. In platinum or white gold, G or H. In yellow or rose gold, I or J, and spend the difference on cut. Buying D when the setting is yellow is paying a substantial premium for a property that the metal immediately conceals.
Clarity, and the only question that matters
Clarity grades from flawless down through VVS, VS, SI to I, and the grading is done at ten times magnification. The relevant question is not the grade, it is whether you can see anything at arm length with your own eyes, which is how the stone will be looked at for the rest of its life.
- VVS1 to VVS2: invisible to anyone. Paying for a certificate.
- VS1 to VS2: invisible without a loupe. The sensible place to be.
- SI1: often invisible, sometimes not. Depends entirely on where the inclusion sits.
- SI2 and below: assume visible, and assume it is under the table where the light goes.
The variable nobody puts on a report is position. An SI1 with a small crystal near the girdle, hidden by a prong, is a better stone than a VS2 with a feather directly under the table. This is the single largest arbitrage available in buying a diamond, and it requires looking at the plot diagram rather than the letter.
Carat, and the cube law
Weight grows with the cube of the linear dimension. Surface area, which is what an observer sees, grows with the square. So a stone that is fifty per cent heavier is only about fourteen per cent wider. Doubling the weight of a round brilliant takes it from about 6.5 mm to 8.2 mm, which is a quarter larger to the eye for twice the weight and considerably more than twice the price.
On top of that, prices step sharply at the round numbers. A 0.90 is roughly 6.2 mm and a 1.00 is roughly 6.4. The step across that boundary is commonly fifteen to twenty per cent for two tenths of a millimetre. Buying at 0.90, 1.40 or 1.90 is the cheapest available upgrade in the entire category, and the difference is invisible without callipers.
How to spend a fixed budget
Take whatever the number is. Fix cut at excellent and refuse to move it. Set colour by the metal, G to H in white, I to J in yellow. Set clarity at VS2, or at SI1 if you have looked at the plot and the inclusion is peripheral. Then buy whatever weight is left, just under a round number.
Done that way a budget buys a stone that performs in the low light where it will actually be seen, which is the entire point of the object. Done the other way it buys a number to say out loud.
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