Carat

Diamonds are weighed by the standard measure of carat.  The carat weight is a measurement of weight, not size.

The carat is a unit of weight, which derives from the carob seed.  The pods of the carob, or locust, tree contain tiny seeds, which are remarkably consistent in weight.  These seeds were used by early gem traders to weigh their diamonds.

A 1-carat diamond used to equal the weight of one carob seed, but in today's terms the carat is a standard weight of 0.2 grams, and each point is divided into 100 points.  So for example, a quarter carat is 25 points, written as 0.25ct; a half carat is 50 points, written as 0.50ct, and so on.

It is easy to weigh a loose diamond by using a carat weigh scale, but once it is mounted it is only possible to estimate the weight by using special gauges and formulae.  Several 1-carat diamonds may look different sizes because of the way they were cut.  Same as how some stones that are 0.90ct may appear to be a full carat.

Diamond weight can also be described in fractions.  Although fractions are not the most accurate way of describing weight, they are very common terms.

Common Methods of Describing a Diamond's Carat Weight

1 ct	1.00ct	100 points
3/4 ct	0.75ct	75 ponts
1/2 ct	0.50ct	50 points
1/3 ct	0.33 ct	33 points
1/4 ct	0.25 ct	25 points
1/2 ct	0.20 ct	20 points
1/5 ct	0.15 ct	15 points
1/10 ct	0.10 ct	10 points

Diameters and Corresponding Weight of Round, Well-proportioned, Brilliant-cut Diamonds.

diameter in MM	approx. weight	diameter in MM	approx. weight
1.3	0.01	4.00	0.23
1.7	0.02	4.01	0.25
2.0	0.03	4.5	0.33
2.2	0.04	4.8	0.40
2.4	0.05	5.2	0.50
2.6	0.06	5.6	0.56
2.7	0.07	5.9	0.75
2.8	0.08	6.2	0.85
2.9	0.09	6.5	1.00
3.00	0.10	7.00	1.25
3.1	0.11	7.4	1.50
3.2	0.125	7.8	1.75
3.3	0.14	8.2	2.00
3.4	0.15	8.6	2.25
3.5	0.16	9.00	2.50
3.6	0.17	9.3	3.00
3.7	0.18	10.2	4.00
3.8	2.00	11.0	5.00