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How to Calculate Gemstone Weight

Learn how to determine the weight, in carats, of precious gem stones to know their value. The number of carats a diamond or ruby contains is a measure of its weight not its size or cut. A gemstone's carat value most directly impacts its value. Specifically, one carat equals 200 milligrams. Normally jewelers use a very precise scale to determine gemstone weight, in ounces or milligrams, before converting to carats. If you don't have a high-precision scale, use Archimedes' Principle -- a water displacement method -- to find the gem's volume and then calculate its weight.

Things You'll Need

Graduated cylinder
Calculator

Instructions

- 1
  Fill a graduated cylinder to the 10 mL level with water. This is the initial water level.
- 2
  Completely submerge the gemstone in the water. Record the new, higher water level of the graduated cylinder. For example, assume a final water level of 10.25 mL.
- 3
  Subtract the initial water level from the final water level to obtain the volume of the gemstone in milliliters. Completing this step, for the example, you have 10.25 mL minus 10 mL, or a volume of 0.25 mL. One milliliter equals one cubic centimeter, so the volume of the gemstone is also 0.25 cubic centimeters.
- 4
  Multiply the density of the gemstone, in grams per cubic centimeter, by the volume to arrive at its mass in grams. Assume, for the example, the gemstone is a diamond with a density of 3.51 grams per cubic centimeter. Continuing the example leads to 3.51 grams per cubic centimeter times 0.25 cubic centimeters, or a gemstone mass of 0.88 g.
- 5
  Divide the gemstone mass by 0.2 to convert to carats, because 0.2 grams equals a single carat. Completing the example, you have 0.88 g divided by 0.2 grams per carat, or a gemstone weight of 4.4 carats.

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