CARAT WEIGHT AND SIZE
The weights of diamonds and most other gems are expressed in metric carats. One metric carat equals 0.200 grams (200 milligrams or 1/5 gram). A carat is divided into 100 equal units called points. According to US Federal Trade Commission rulings, all weight representations are subject to a 1/200 carat (one-half point) tolerance. If other factors are equal, the more a stone weighs, the more valuable it will be.
It's diamond industry practice to weigh diamonds to a thousandth of a carat, then round the weight to a hundredth of a carat. You round the weight up only if the number in the thousandths place is 5 or more. For example, you'd round a diamond that weighs 1.764 cts. to 1.76, but one that weighs 1.765 cts. to 1.77.
THE MILLIMETER GAUGE
The millimeter gauge is an excellent tool for measuring diamonds. Not only can it measure the dimensions of loose diamonds, it can also meas¬ure mounted diamonds as long as the mounting has openings in strategic places.
To accurately measure the depth of a diamond in an open-back mounting, make sure the millimeter gauge's outer jaws contact the top and bottom of the stone (top left). With a prong-set stone, measure diameter between the prongs (top right). With a bezel-set stone, estimate where the bezel facets come to a point at the girdle and measure across the stone at those points (bottom).
The dial on a millimeter gauge shows tenths of a millimeter and whole millimeters up to 10. Each time the needle makes a full rotation, you have to add 10 millimeters to the measurement shown. You also have to estimate measurements to hundredths. There are gauges with digital readouts that display measurements to hundredths, eliminating the need for estimation.