Unit of Weight
Besides color, clarity and cut, weight provides a further basis in the valuation of a diamond.
The weight of all gemstones is measured and counted in carats, commercially abbreviated to "ct". According to CIBJO regulations the carat weight noted: "The weight of a diamond is to be indicated to the second decimal point; the international abbreviation is ct."
It is only since the beginning of this century that the carat weight has been adapted to the metric system and is now used-internationally. It is today standardized just as the 10 x magnification, and the use of master stones for color determination of diamonds.
The unit of weight is established as follows:
one metric carat = 200 mg = 1/5 gram.
The carat is subdivided into tenth, hundredth and lately even thousandth. Very small brilliants are measured in "points"; one "point" is equal to one hundredth of a carat (0.01 ct).
The designation "melee" (French: mixed) comprises the group of stones of sizes from 0.07 to 0.14 or 0.15 ct. Stones between 0.12 and 0.15 ct are termed "coarse" melee.
A carat is also subdivided into "grains" - a unit of weights used for pearls where
one carat = four grains.
A one-carater therefore is also a four-grainer. The term "one-carater" does not only refer to a diamond of exactly 1.00 ct, but comprises diamonds within a certain weight zone, which can be described as follows:
| one-carater (four-grainer) | 0.95-1.05 ct |
| three-quarter carater (three-grainer) | 0.72-0.76 ct |
| half -carater (two-grainer) | 0.47-0.56 ct |
| quarter-carater (one-grainer) | 0.23-0.26 ct |
Commercially, however, diamonds of one group are priced differently: a brilliant of 0.99 ct is roughly 10-15 % cheaper than a brilliant of 1.01 ct of the same quality.
This is the reason why diamonds are often cut "heavy", to bring the
maximum weight with loss of quality of cut. The experienced grader
takes into account the proportion of a stone especially in such weight
border-line cases; specifically the girdle must be observed closely, as
a thick girdle can increase the weight of a stone by a few points.
The carat price is hot uniform for all sizes of a certain quality, but rises progressively with the size of the stone. The rarity of larger stones thus also influences the price formation. The reproduced diagram illustrates the rough progression depending on the market, i.e. supply and demand of certain stone sizes (Fig 388).
| Aids and Instruments for Cut-Grading | Weighing |