Carat is a term that refers to the weight of a diamond. Before the twentieth century, diamonds were measured by using a carob seed. Carob seeds are small and uniform and served as a perfect counter weight to a diamond being measured. Thus, it is the word "carob" that sits at the origin of the word "carat" we use today.
Diamond Size and Diamond Carat Weight
The size of a diamond is proportional to the carat weight of a diamond. When rough diamonds are cut and polished into finished diamonds, as much as 2/3 of the total carat weight may be lost. Since larger rough gems of high quality are found less frequently than smaller rough gems of high quality, a single two carat diamond will be more expensive than 2 one carat diamonds of the same quality.
In the United States, the majority of diamonds used in jewelry and sold as loose diamonds are one carat or less in weight. The average engagement ring diamond sold in the U.S is less than 1/2 carat in weight.
A diamond will increase in weight much faster than it increases in actual "face-up" diameter. For example: while an ideal cut one carat diamond measures approximately 6.5mm in width, a diamond of twice its weight measures only 8.2mm wide - less than a 30% increase. The graphic below will help illustrate this point
Which Carat Weight Is Right For You?
This question has no direct answer. It really depends. When you look at a diamond engagement ring all you really see is the size of the surface area on the top of the diamond. You don't really get an idea about the diamond in terms of carats. So which carat weight is right for you depends on your budget and desire. Although weight clearly does matter because it influences cost quite a bit, you should consider focusing on diamond cut grade and diameter.