How to Measure the Points & Carats of Diamonds

by Cam Merritt

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.

Step 1

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.

Step 2

Remove the diamond from any setting. You can't accurately measure a diamond while it's set.

Step 3

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.

Step 4

Place the diamond on the scale and read the weight.

Step 5

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.

Step 6

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.

Tips

  • 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.

About the Author

Cam Merritt is a writer and editor specializing in business, personal finance and home design. He has contributed to USA Today, The Des Moines Register and Better Homes and Gardens"publications. Merritt has a journalism degree from Drake University and is pursuing an MBA from the University of Iowa.