The Grader
Carat
Carat measures weight, not size — and weight, more than any other C, drives price. Where the unit comes from, and the formula that catches an imitation.
Carat is the measure of weight for diamonds — abbreviated ct, where 1 ct = 0.20 grams. Very small brilliants are measured in points (1 pt = 0.01 ct), and weight is sometimes given in grains (1 ct = 4 grains). The word has nothing to do with size directly: two stones of equal weight can look quite different depending on how they are cut.
Caraters and parcels
In the trade, a “carater” is a band of weight rather than an exact figure:
| Carater | Weight range |
|---|---|
| 1 ct | 0.95 – 1.05 ct |
| ¾ ct | 0.72 – 0.76 ct |
| ½ ct | 0.47 – 0.56 ct |
| ¼ ct | 0.23 – 0.26 ct |
When buying loose small goods, one buys mêlé parcels — French for mixed — typically diamonds from 7 to 15 points. A parcel of only the larger 12-to-15-point goods is called coarse mêlé.
Size and the rarity premium
Larger diamonds are far rarer in nature than small ones, and price per carat rises steeply with weight. A single two-carat stone is worth considerably more than two one-carat stones of the same quality, precisely because the larger rough it came from is so much scarcer.
The authentication test
Comparing a stone’s weight with its measurements and density helps prove it genuine. The Scharffenberg formula is a fine tool for this. For a round brilliant:
ct weight = height (mm) × Ø² (mm) × 0.0061
If the calculated weight matches the weighed weight, the stone is not an imitation. Density confirms it: measured on a hydrostatic balance, the specific gravity of diamond is 3.52.
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