More Carats Does Not Mean Bigger
Why Carats & Size Are Not The Same

Historically, the carat measurement varied greatly from geographic area to geographic area. In 1907 the gemstone measurement was standardized as the metric carat that weighs 200 mg, or 0.2 grams.

Because gemstones vary so greatly in their densities or atomic weights, the size of a one-carat gemstone can be very different than a one-carat stone of another gemstone type.

For example, all diamonds are made of carbon so therefore they all have the same density, or atomic weight. We know that a 1-carat round brilliant diamond will be 6 to 6.1 mm in diameter. However, colored gemstones are a combination of elements and each of those elements have a different density/atomic weight.

Amethyst, which has a density of 2.65 will, will produce a larger-sized gemstone weighing one carat than a ruby, with its heavier density/atomic weight of approximately 4.00, will.

The size of a gemstone is normally noted in millimeters. The size measurements taken with the carat weight, gives a much more accurate picture of a gemstone.

Three additional units of measure have been used to describe gemstones by the jewerly industry. The gram weight measure is used to weigh less precious gemstones and gemstone rough. The grain (approximately 1/4 carat) is the historically the measurement unit for pearls. Pearls are being more frequently measured in carats today. Momme is an old Japanese measurement for cultured pearls and is rarely used today.