The most common way to weigh cut gemstones or cabochon stones is by the carat. A carat is 1/5 of a gram, or there are 5 carats to a gram. This standardized method of weighing cabochon stones was suggested in 1907 by the International Committee on Weights and Measures, and was officially adopted in the U.S. in 1913. Most other countries also adopted it from 1908-1930.

A United States copper penny weighs 15.5 carats.

Originally gemstones were actually weighed using carob seeds! So, the word "carat" originates from the carob seeds historically used as a standard for weighing gemstones due to their relatively uniform weight.

The word “carat” should not be confused with “karat”, which is used to describe the pureness of gold. The number of karats represents the ratio of gold to other metals in an alloy. 24 karat gold is the purest so a karat is one part of 24. This means that 18 karat gold is 18 parts pure gold and 6 parts other metals.

Pearls are typically measured in carats or grams, but “grains” can be used for smaller pearls. There are 4 grains to a carat or a grain is equivalent to approximately 0.0648 grams.

Most cut gemstones, whether faceted or cut into a cabochon, are measured in size by millimeters.  A millimeter is about .04 of an inch in length. There are about 25 millimeters to an inch.  Often gemstones are cut in calibrated sizes. A calibrated gemstone or cabochon stone is one that has been cut to fit standard size commercial gemstone jewelry mountings and settings. For example, a 30 mm by 40 mm oval shape gemstone, or a 6 mm in diameter round shape gemstone or cabochon.

A United Sates copper penny is about 19 mm in diameter.

Gemstones have different densities, and therefore 2 stones of the same size may have different weights. For example, a 6 mm round diamond is almost half the weight of a 6 mm round sapphire. Precious gemstones are almost always sold by weight. Semiprecious stones usually are priced individually more by size, as well as color, pattern, and rarity of the cabochon stone. Cheap or lower grade semiprecious gemstones sold in bulk may be priced by the gram or carat.

Rough gemstones may also be weighed by carats if they are smaller or have a high value. But rough semiprecious gemstones are usually sold by the gram, by the kilo (about 2.2 lbs.) or by the pound in the United States.