A carat is a unit of measurement used almost exclusively with gemstones. Many people confuse carat with karat, which is a different type of measurement used in the jewelry industry to indicate the percentage of gold used in a metal alloy (such as 14-karat and 18-karat gold). Contemporary carats are equal to 200 milligrams, which is about the same weight as a standard paper clip. You will usually only see the term carat used to describe the size and weight of cut gemstones. This is used especially with diamonds.


The term ‘carat’ was first used in the 1400s, and is derived from the Italian word for carob seed. These seeds were reputed to experience a very low range of variability in mass, whereas different species of seeds can be considerably larger or smaller, or more or less dense than other seeds of the same type. In other words, every carob seed is almost exactly the same size and shape as every other carob seed, making them perfectly-suited as a reference of measurement.

The United Kingdom Board of Trade is responsible for promoting the carat as a standard unit of weight and measurement for gemstones. They were also the first to give it a scientifically precise measurement designation.

Use in Gemstones

Since the late 16th Century, carats have been used almost exclusively for designating the size and weight of gemstones. The 5 C’s of Diamonds, which determine diamond quality and value, include carat weight (the other C’s are cut, color, clarity, and certification). Most valuable diamonds, assuming the other elements are of high standards, are over one carat in weight. Smaller diamonds, or those less than one carat, are much more common and therefore more affordable.

The vast majority of diamonds weigh in at around one to two carats. Larger stones are extremely rare and are often included in museum collections (such as the Hope Diamond and the Star of India). A 100-carat, flawless diamond is known as a paragon, of which there are only a handful in the entire world. Other stones, such as rubies, sapphires, emeralds, etc. can be found in larger sizes due to their lower, overall value. It is not uncommon to see five carat rubies on the market.

It is important to understand carats when shopping for jewelry, especially anything as important as an engagement ring, for example. Gemstone weight is an important factor in determining value.

Add Gemstones To Your Collection

When you are ready to add to gemstones to your collection, visit U.S. Auction Online to make a bid, or you can register to view winning bids to get an idea of the value of gemstones that you already possess.