Carat
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.

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

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

Grain:
(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 Ever more superceded by carat weights, the old Japanese weight- the Momme (=3.75 grams = 18.75 carats) is barely used any more in Europe.

WEIGHT CONVERSION TABLE 

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

STANDARD ABBREVIATIONS

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