Diamonds are evaluated using what the jewelry industry calls the "four C's"--color, cut, clarity and carat weight. You may have to be a jeweler to accurately assess the first three, but all you need to determine carat-weight is a good scale capable of measuring to the milligram. When measuring diamonds, 1 carat is equal to 200 milligrams, or two-tenths of a gram. Each carat is divided into 100 "points" of 2 milligrams apiece. So a 75-point diamond is 0.75 carats, or 150 milligrams.
Find a scale capable of measuring milligrams. These are often referred to as milligram scales, gem scales or jewelry scales. See the Tips below for more information.
Remove the diamond from any setting. You can't accurately measure a diamond while it's set.
Calibrate the scale. The weights involved here are so small, a scale that's just a little "off" could give you a highly inaccurate reading. Instructions for calibration should be included with the scale; some even come with metal blocks of precise weights to aid in calibration.
Place the diamond on the scale and read the weight.
Do the math. Subtract 200 milligrams for each carat. Take the remainder and divide it by 2 for the remaining points. For example, if a diamond weighs in at 320 milligrams--lucky you--that's 1 carat (200 milligrams) and 60 points (120 milligrams divided by two), or 1.6 carats.
Take the diamond off the scale and repeat the process, starting with Step 3. Twenty milligrams is exceedingly small--less than 1/100th the weight of a dime--so the more times you calibrate and measure, the more confident you can be about the weight.
Milligram scales are available online for less than $100. See the link in the Resources for some examples. The weight range of a precision scale is expressed in the format "A x B grams," with A equal to the heaviest item it can measure, and B being the lightest. Look for a B value of .001 grams. Gem scales are often 20 x .001 grams.
You also can simply ask a jeweler to use her scale. If you need a diamond removed from its setting, you may want to go to a jeweler anyway.