Carat

Carat weight refers to the actual physical weight of a diamond.  Diamonds are weighed in metric carats – one carat is equal to .20 or 1/5 of a gram or five carats per one gram. 

The term carat is derived from the word carob.  Early traders, in need of a uniform unit of weight, used carob seeds because of the uniformity of their size and weight.  Eventually, carob evolved into carat.

For diamonds that are one carat or less, most jewelers refer to carat weight in terms of points.  For example, one carat equals 100 points of weight, ½ carat equals 50 points, and ¼ carat equals 25 points.

As stated above, the term carat refers to the physical weight of a diamond, not the size of a diamond.  Since there can be many variances in a diamonds physical dimensions, two or three one carat stones may weigh the same yet look very different. 

In the jewelry industry, diamond prices are quoted in terms of price per carat.  The prices increase with the size of the diamond, because larger diamonds are more rare and their value is higher.  A one carat diamond is not necessarily twice as expensive as a half carat of the same quality, but can be up to five times the price per carat of the ½ carat diamond simply because larger diamonds are more rare in nature.