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Home > Gemstones > Units for weighting Gemstones
Units for weighting Gemstones

This weight is used in the gems trade since ancient times. The name is derived from the seed (kuara) of the African Coral tree or from the kernel (Greek-kertion) of the Carob bean. Since 1907, Europe as well as America, have adopted the metric carat of 200 mg. or 0.2g. Therefore weights given for famous old diamonds often vary because local carats and not metric carats were used. The carat is subdivided into fraction (1/10 ct) or decimals (1.25ct) up to two decimal places. Small diamonds are weighed in "points" = 1/100 cts. (=0.01 cts.). The table below illustrates diameter and corresponding carat weight for diamonds cut in the modern brilliant cut. Gems with different specific gravities and different cuts obviously have different diameters.

Carat Weight

The carat weight of gems is not to be confused with the karat used by goldsmith. In the case of gold, the karat is not a weight measure, but shows purity. The higher the karatage, the higher is the content of gold in the piece of Jewellery. The weight can differ

Weight used in gems trade for lesser gemstones are for rough gem stones (for example, quartz)

(Latin - granum) Weight measure for pearls. Corresponds to 0.05 grams = 0.25 or 1/4 carat. Ever more superceded by carat weights, the old Japanese weight- the Momme (=3.75 grams = 18.75 carats) is barely used any more in Europe.


Unit Grams Carats Oz. Troy Dwt. Oz. Av. Grain
1 Carat .20 1 - - .00705478 3.08647
1 Gram 1 5 .03215 .6430 .3527 15.4324
1 Penny Weight 1.5517 - 0.5 1 .548571 24
1 Grain .6479 .3240 .0020833 .041667 .0022857 1
1 Ounce Avoirdupois 28.3495 141.75 .9114883 18.2297 - 437.5
Troy 373.241 1,866.12 12 240 13.1657 5.760
1 Pound Avoirdupois 453.295 2,267.96 145833 291.667 16 7.000


Avoirdupois Av. Milligram Mg.
Carat C. or Ct. Millimetre Mm.
Grain Gi Penny weight Dwt.
Gram G Pound Ib.
Kilogram Kg. Troy T

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