What is a "carat"?

Everyone talks about Carats when discussing gemstones, and most understand that it relates to the size of the gemstone.  But there's more to "Carat" than size alone.

*According to the Gemological Institute of America (GIA), and to put it simply, diamond carat weight measures how much a diamond, or any other gemstone for that matter, weighs.  And while carat is specifically about weight, it logically follows that the more a diamond or gemstone weighs, the larger the gemstone.  Here we'll discuss carat relative to diamonds.
A metric “carat” is defined as 200 milligrams. Each carat is subdivided into 100 ‘points.’ This allows very precise measurements to the hundredth decimal place.  A jeweler may describe the weight of a diamond below one carat by its ‘points’ alone. For instance, the jeweler may refer to a diamond that weighs 0.25 carat as a ‘twenty-five pointer.’ Diamond weights greater than one carat are expressed in carats and decimals. A 1.08 carat stone would be described as ‘one point oh eight carats.’
All else being equal, diamond price increases with diamond carat weight because larger diamonds are rarer and more desirable. However, two diamonds of equal carat weight can have very different values (and prices) depending on three other factors of the diamond 4Cs: Color, Clarity, and Cut.
While now you know what carat means, it’s also important to remember that a diamond’s value is determined using all of the 4Cs, and not just carat weight.

We mentioned that this discussion is about the carat weight of diamonds.  While it's true that the carat is used to weigh other gemstones as well, it's important to understand that a 1-carat diamond may be larger or smaller than a 1-carat sapphire, emerald, or ruby because of the difference in chemical composition and density.  Too, other colored gemstones have their "equivalent" of the 4Cs, those two of defined differently for different gemstones.

*Thanks to GIA for their definition of carat from their website.