Diamond Carat Weight
A carat is a unit of weight/mass measurement and 1 carat is equal to 200 milligrams (0.2 grams, 0.007055 oz). This is the way diamonds and other gemstones, including pearls, are measured. A 2 carat diamond will therefore weigh 0.4 grams, although the actual size will vary depending on the style of cut (see chart below).
As far as value goes, larger diamonds are more rare than smaller ones, which means the cost per carat increases with the size of the diamond and overall weight. All other things being equal, i.e. color, cut and clarity, a 2 carat diamond will always be worth more than two 1 carat diamonds.
When using carat weight to determine the value of a diamond, it is also important to consider the other factors and not use weight alone. When considering a piece of jewelry with more than one diamond, the total carat weight is often used.