All About Carats/Karats

 What Is a Carat?

Well, a carat is a root vegetable that...No, sorry, we're not talking about "carrots."  The term carat refers to the generally accepted unit of measure for the mass ("weight" for common purposes) of a diamond.  The term carat is often abbreviated as "CT."  Each carat is divisible into 100 units called points.  One point equals 1/100 (0.01) of a carat.  One carat equals

  • 100 points
  • 1/5 gram
  • 200 milligrams

What Does "CTW" or "CTTW" mean?

When there are multiple diamonds or gemstones in a piece of jewelry, the total of the carat weights of all the stones are referred to a Xcarats total weight.  Carats total weight is abbreviated at CTW or CTTW.  Most jewelers have a tolerance for variation in the total carat weight established.  If the weight is approximate, the jeweler is required to specify the accepted variance.

Why aren't diamonds just weighed in grams?

Good question.  Diamonds are generally sold by their weight (mass).  A small fluctuation in weight can mean a large difference in price.  Therefore, one gram is too large of a measurement to use for the weight of diamonds.  A carat, being equal to 1/5 of a gram, is much more precise.  Diamonds weights are carried out to the second decimal place (e.g. 1.75 carats).

Does that mean all diamond weights are really just rounded?

Well, not necessarily all.  A diamond can certainly weigh, for example, 1.000 carats.  In the United States, the Federal Trade Commission (FTC) allows for diamonds to be rounded up to the next number in the second decimal place if the number in the third decimal place is a 5 or higher (e.g. 1.745 carats can be rounded to 1.75 carats).  However, GIA and many diamond dealers have set their own standard that diamond weights are only to be rounded up if the number in the third decimal place is a 9 or higher (e.g. 1.749 carats can be rounded to 1.75 carats).  This means that anything weighing, as in our example, 1.748 carats or less is rounded to just 1.74 carats.  In this way, the diamond consumer pays the fairest price for the diamond based on its weight.

So gold is weighed in carats too?

No.  Although you have undoubtedly heard terms "14 karat gold," "18 karat gold," etc., a diamond's carats have nothing to do with a gold's karats.  The term karat (with a "K" in the United States) refers to the purity of gold.  Pure gold (considered 99.9% gold or purer) is rated as 24 karats.  This is the highest possible purity rating.  When gold is referred to as "18 karats" (or "18K"), the gold is 18 out of a possible 24 karats.  This comes out to be 75% purity, and this is why some 18K gold is marked as "750" (referring to 75.0% gold content).  The other 25% is an alloy of other precious and non-precious metals used to give the gold greater strength and different color (e.g. "white gold" and "rose gold").  To make matters even more confusing, in Great Britain, for example, "karat" is spelled with a "C" making it "carat."  Therefore, there is no spelling difference between the unit of weight and the unit of purity.

Is gold really sold as low as 1 karat?

Not in the United States.  In the US, the FTC limits the sale of gold to 10 karats (58.5% gold) and purer.  Anything less than 58.5% gold cannot be legally referred to as "gold"  (The legal limit for platinum is 90.0% platinum).  Again across the pond in Great Britain, gold can be sold in as low as 9 carats (37.5% pure gold; remember the difference in spelling!).  In Italy, the minimum purity level allowed for domestic sale is 18 karats (75.0% pure gold).

Originally posted by me on on 4/7/2010.