Gemstones are weighed in metric carats. One metric carat (ct) equals 0.200 grams (1/5 of a gram or 200 milligrams). A carat is further divided into 100 units called points. Reputable jewelers and Gem Trade Laboratory apply the same minimum standard of accuracy to colored stones as the US Federal Trade Commission (FTC) rules require for diamonds: All stated weights must be accurate to 1/200 .ct (one-half point).
Weight is usually written as a decimal fraction (e.g., 1.35 ct, or 0.73 ct), and should always be expressed to the second decimal place. Follow the international trade practice of rounding diamond weights up to the next higher point only when it is within 0.01 carat (rounding up only from 0.009 carat). They apply the same standard to colored stones, for consistency. Thus, a 0.478-carat stone is rounded down to 0.47 carat. Only a 0.479-carat stone may be rounded up to 0.48.
Generally speaking, the more a gemstone weighs (the larger it is), the more rare it is and, therefore, the more expensive it is likely to be per carat. Thus, one two-carat ruby is worth more than two one-carat rubies of the same quality. But stones that exceed sizes that are attractive and relatively easy to mount in jewelry are usually lower in per-carat price than the largest jewelry-size stones. The size limit actually depends more on measurements than on weight (usually 20 mm in the smallest face-up dimension).
Some gems, like blue topaz and citrine, are likely to occur in large sizes. Others, like ruby, are rare in sizes over three carats.