Getting the Carat weight of gems correct

If you are in the gemstone market, apart from the obvious colour and size, there is one more very important thing that you should evaluate. And nobody is going to cheat you on this if you know how to calculate the size of a gemstone. We all it- The Carat. When it comes to finding the right sized gemstone within a budget you can handle, carats is something you should get familiar with.

What is a carat?

Gemstones are weighed in carats. Every gemologist would count the carat as the basic factor to classify different gemstones. The raw pieces are cut into smaller and smoother edges. The finished gemstones are tagged with the carat after cutting to measure up the amount of wastage in the processing.

Abbreviated to “ct” in a gemstone parlance, a carat is a unit of measurement usually represented in points. A point of a gemstone size is 0.01 of a carat or in general terms, one unit is 1% of what we know as a carat. For very small garnets, rubies and travertine, the usual unit of size determination is a “point” and not carats.

Carat and pricing:

The word “carat” has same origin as that of the word “keratin”. Keratin is derived from the Latin word “keratos”, meaning “little horn”. The carob tree seeds are called keratin too, and the gemstones resemble them in more than one way that you can think of. Carats signify the uniformity of different sized gemstones.

As far as pricing is concerned, a gemstone with higher carat value is quoted at a higher price.

A correlation: Grams versus carat

Until the end of World War I, gemstones were still measured in grams and not carats. After 1920, the popularity of carats caught up owing to bigger sized diamonds being cut into cabochons and smaller shapes. As a method of approximation, gem scientists and lapidary artists zeroed on to carat as an international standard.

One carat = 1/5th of a gram

In general term, one carat is 20 percent of what we know as gram weight.

Law of gemstone pricing:

Price per carat is the universal factor to identify what the gemstone might be weighing. The pricing is fixed as per a very popular hand-rule called “Indian Law” or “Tavernier’s Law”.

In a bid to find the weight of the gemstone in terms of carats, the formula used is:

Carat = Facet style factor (ff) x {Girdle length (grl) x Girdle width (grw)} x depth (dep) x Average specific weight

These days, a specially designed digital scale is used to weigh precious gemstones. The same scale can derive the combined carat weight of more than one gemstone.

To convert a gemstone’s weight in milligrams into carat, divide it with a factor of 200.

For example, if a diamond weighs 250 milligrams, the weight in carat is: 250/200= 1.25 ct

In terms of points, this carat value is equal to 125 points.

Apart from carat of course, there are three C’s you need to define to arrive at the right price- Cut, Clarity and Colour.


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