THE FOUR FAMOUS "C". THE CARAT

From among all the weight units, the most commonly used is CARAT, a word that derives from the Arab word KIRAT, the weight of the carob tree seed, which was used as a measurement unit because the seeds’ weight was always the same.

CARAT

•       It is the weight unit usually used for precious stones.
•       Equivalent to a fifth of a gram: 1ct = 0.2 g = 200 mg
•       These days the carat metric is used in all countries.

The weight in carats of precious stones must not be confused with the number of karats as an expression of the GOLD GRADE (quality index that expresses the amount of fine gold that a certain jewellery piece contains).

24 karats =  100%  //   18 karats  - 75 %.

Although this is the most used unit, it is also important to get to know other terms:

POINT. Weight unit frequently used for small diamonds.

1 POINT =  1/100 carat  = 0.01ct.

GRAIN. Unit traditionally used for pearls and diamonds, although nowadays it is being replaced by the carat or the point.

1 GRAIN = 1/4 carat  = 0.25 ct.

GRAM.  Weight unit used for rough material and precious and ornamental stones of a lower value.

MOMME. Japanese unit that is hardly used on the European pearl market.

1 MOMME = 18.75ct = 3.75 g

It is important to know that the price of precious stones increases exponentially as the size increases. This is due to the RARETY factor. To find a diamond of 1 carat with perfect characteristics is very hard, but to find one of 2 is much harder, and so on. 

I will give you a clear example:

1 ct. diamond, Color D, Clarity IF, value: 21.700 $/ ct

2 ct .diamond with the same characteristics has a value of 52,000 $/ ct., so its final value would be $ 104,000 

3 ct. diamond,  97,000 $/ct, would be valued at $ 291. 000.

It is clear that the price increases exponentially, but what happens with the size?  Not so much.

Below you can find a link where you can see the relation for carat / size. There you can see how a 1 ct. diamond usually has 6.50 mm diameter, whilst a 2 ct. diamond has a 8.20 mm diameter and a 3 ct. diamond, 9.40 mm.

https://www.diamonds.pro/ring-comparison/?carat1=1&shape1=round&length1=&width1=&depth1=&carat2=1&shape2=princess&length2=&width2=&depth2=

Another relationship to be aware of is: CARAT / CUT.  Very often gem cutters try to use as much as possible of the rough stone, at the expense of the cut quality. This means that the stone won't be well proportioned, so on some occasions a window effect (pavilion with low depth), or darkness and shadows in the centre (very deep pavilion)  will be noticeable, which will diminish the beauty of the stone.