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How to Size Faceted Gemstones

Faceted gemstones have been cut and polished into showroom condition and are sold to consumer by their weight -- expressed in carats. One metric carat is equal to 200 milligrams. Analog and digital scales are used to weigh the gem, determining its carat size and giving the gem a value range. Because each gemstone type has a different density and composition, determine the size by the weight. For a quicker sizing, use charts to correlate the gem type, gem shape and measurement to a specific carat amount.

Things You'll Need

Scale that reads milligrams
Gemstone caliper
Gemstone type and sizing chart

Instructions

- 1
  Compare the stone, visually, to the gemstone type and sizing chart to identify the type of stone you're sizing as well as its shape. Common shapes include round, oval, emerald cut, pear-shaped, marquise and square.
- 2
  Squeeze the caliper around over the top of the gem, touching the two caliper arms to the sides of the gem. Note this measurement as the gem's width, in millimeters. For round cuts, this is the only measurement you'll make. For all other shapes, measure the length and width of the gem, and note the dimensions in millimeters. For example, an emerald cut may be 5x3, indicating 5 millimeters long by 3 millimeters wide. With this measurement, you can estimate the size of the gem, in carats, by referencing the sizing chart.
- 3
  Place the gem on the scale to determine its exact weight in milligrams. Multiply the weight by .20 to determine the exact amount of carats in the gem. For example, if the gem weighs 3 milligrams, its size is .6 carats.

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