Carat and Size of Gems

Carat and Size of Gems

Weighing Gems:

In the early history of gem marketing, depending on the geographic location, one of two common items, familiar to both buyers and sellers, was used to measure the amount of gem material being bought and sold: the wheat grain and the carob 稻子seed. Each of these commodities was known for being particularly uniform in size and weight. We still see remnants of this early system in today's terms: "carat" the international metric unit used for gems, and "grain" a unit sometimes used in selling pearls, and also in today's system of apothecary 药剂师measure.

**As we work through this section, you'll probably begin to wonder why it's all so complicated, confusing and haphazard seeming. Unfortunately, the system in place today developed bit by bit from mergers 合并and splits amongst pre-existing local systems. The, sometimes frustrating, result is pretty much of a hodgepodge大杂烩. Although some degree of uniformity has been introduced by the use of the metric system, things still are far from predictable and totally logical.

Carat: The carat, pronounced like the vegetable, carrot, and abbreviated "ct" is 0.2 grams. So, there are five carats per gram. The metric system is the basic international standard used for gem commerce. Many of us who live in the US or UK where English measure is more common, need to take time, and do some practice, in order to get a "feel" for carats, grams, etc. The ounce, a familiar English unit of weight, equals approximately 142 cts. So, there really isn't an appropriately small unit in the English system which could be easily applied to gem weights. [To illustrate: a 1 ct. gem weighs 0.007 oz.]

Another oddity of the US system is our use of the term "karat", also pronounced like the vegetable carrot, but abbreviated "k" or "kt" to indicate the fineness (purity) of gold. In most other countries, the purity of gold is indicated by the number of parts of gold out of 1000, such as 585 or 750, so there is no chance of confusion with gem weights. The number 585 means that 585 out of 1000 parts of the alloy are gold or, in other words, that the gold content is 58.5%.

In comparison, the karat system uses the number of parts out of 24 that are gold. 24k means 24/24th, pure gold, also known as "fine" gold, 18k gold = 18/24th gold, and 14k = 14/24th gold. (14k and 18k and 24k translate then, in the International system, to 585, 750 and 999 respectively). 

**Check the text: See Lyman, pg. 41. The authors (who are Italian and can be forgiven because as Europeans they don't use the karat system, mistakenly use the word "carat" instead of "karat" in describing how gold is marked in the US, and Lyman, the American editor, didn't catch it. So, don't you be confused: For gems it's carat, for gold it's karat !!

[Stamp on a 24k or "fine" gold piece]

The time honored way that jewelers and gold dealers tested gold purity was by using acids and a set of test needles of known karatage. A streak条纹 was made by the object being tested on a stone plate and comparison streaks made below it with the test needles. Then the acid solution was applied to all. Based on the degree and color of the reaction, compared to the test streak reactions, the composition could be closely approximated.

We get our terms "touchstone" and "acid test" from this ancient procedure. Kits工具箱 using this same principle are sold, and still widely used today, although a newer system based on electrical conductivity is becoming popular. In devices of this newer type the test object is immersed in a few drops of electrolyte solution, and then subjected to a current--> its purity level can then be read directly from the scale. 

["Acid Test" kit, electronic gold tester: images courtesy of]

Getting to know the carat:

Below, you see the carat weights of three common objects: since you are likely to be familiar with their approximate weights, this can perhaps help you begin to get a "feel" for the weights represented in carats.

Common items weighed in carats: 

[Small, (1.5" x 2.0") Post-it note = .75 ct., standard bobby pin = 2.8 ct., dime = 11 ct.]

Special Cases:


Pearl Grain: The pearl grain, is .25 grams, so one gram is equal to 4 pearl grains. Thankfully, the only remaining use of this once important measure, is sometimes seen in the sale of natural pearls by weight. Because there is very little commerce today in natural pearls (virtually all pearls on the market are cultured), it is fast becoming obsolete.

Many cultured pearl wholesalers still sell bundles of pearls in larger units called "momme姆米" which, historically, weighed 75 pearl grains.

Cultured pearls are sold by diameter (millimeters) if they are round, or near round, and by carat if they are oddly shaped (baroque).