What Does the Word Carat Have to Do with Carob Seeds?
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Posted on 05 Feb 2013 02:30

Although you may not realize it, the "weights" we use must be linked to a physical reference object, even today. For instance, the kilogram is represented by the international prototype kilogram, which was sanctioned in 1889. The actual weight is a small cylinder made of platinum and iridium and is held by the Bureau International des Poid et Mesures, who grants access to it and the official copies of the weight.1 There was a time, however, when in order to weigh anything you needed a standard unit of known weight in your possession. Believe it or not, carob seeds used to fill this role, and the word carob is connected to the word carat, pertaining to diamonds.

When the only way to weigh something was with a balance scale, the unknown weight of an object had to be measured by counting the number of unit weights needed to counterbalance it. It was once thought that carob seeds had such uniform weight that they made perfect unit weights!

The carob tree, Ceratonia siliqua, St. John's Bread, or locust tree grows in the Mediterranean and has fruit pods that contain multiple seeds. It was a simple task to get your hand on a bunch of carob seeds of the same size, and count on them all to be pretty much the same weight. Or, at least, it was perceived so. The weight of an object was then expressed in terms of the equivalent number of carob seeds. This was standard for commerce and it was even used to weigh things like gemstones and gold.2,3,4


carob pods growing on Carob tree (locust tree)

Carob Pods on Tree

carob pods growing on Carob tree (locust tree)

Carob Pods on Tree



A carat is a modern unit of weight for gemstones like diamonds. This is not to be confused with karat which is a measure of gold purity. A carat is defined as exactly 0.20 grams. This means that a diamond weighing 100 carats would be 20 grams. Well, according to some, a typical carob seed is about 0.20 grams (200 mg), so the same diamond would weight just about the same as 100 carob seeds. Yep, the modern word carat owes its derivation to the same pathway that named the carob, which was originally used by Arab jewelers to weigh their precious metals and stones. You can see the resemblance in its Greek name keration and its Latin name Ceratonia.2,3,4Although carob seeds would never be acceptable by today's standards and according to this paper, they are no more consistent in mass than a lot of other seeds.5

Although it seems that carob seeds were used as a standard of weight, they were probably also used as a good way to fleece people, as it would be pretty simple to keep a heavier set and a lighter set on hand in order to use the one that suited your needs at the time. If you were selling, the lighter set would fit the bill. If you were buying, then the heavier set would be to your advantage.


carob seeds closeup on white background with ruler

Carob Seeds
image Mihailo Grbic via wikimedia

carob seeds closeup on white background with ruler

Carob Seeds
image Mihailo Grbic via wikimedia



Some texts get confused by the word carat being similar to carrot and so state that it was carrot seeds that were used as weight units. The History Channel show Modern Marvels recently made this mistake as well.

What is the Term Karat Used for Gold?

As mentioned above, the word karat, as used for gold refers to the percentage of gold versus the percentage of an alloy mixed with the gold. The purest gold is .9995+ fine and is called proof gold. Gold of this purity, in the jewelry world, is used for standardization purposes, but pure gold is too soft to be used for jewelry and is very easy to abrade. Therefore gold must be mixed with varying percentages of other metals to make a cheaper alloy that has the desired hardness and quality. Also, other metals are mixed in to change the color.

Since it would not be good to just let people put cheaper alloys in gold willy-nilly without some sort of standard as to what kind of quality you might be getting, the percentage of gold in all gold alloys is standardized. There is not a standardization for how much of any other metals the alloy contains. Importantly, in Britain, the term carat is used both for the meaning given above and for the percentage of gold in a gold alloy, which is very confusing.

The proportion of gold in a gold alloy is termed it's karat content, which is usually abbreviated K. So, 18K indicates a specific proportion of gold. 24K is pure gold, or fine gold. These karat designations actually represent the gold content as how many parts, out of 24 parts, is pure gold. Hence 24K is 24 parts pure gold out of 24 parts, and 18K is 18 parts pure gold out of 24 parts. Here are the percentages of gold in all karats from 24 to 1:

Gold Karat Percentage of Gold
24 100*
23 95.83
22 91.67
21 87.50
20 83.33
19 79.17
18 75
17 70.83
16 66.67
15 62.50
14 58.33
13 54.17
12 50
13 54.17
12 50
11 45.83
10 41.67
9 37.50
8 33.33
7 29.77
6 25
4 16.67
3 12.50
2 8.33
1 4.17

* As close as you can get to 100% pure, as in the .9995+ pure given above.

References
1. "International Prototype of the Kilogram." BIPM. N.p., n.d. Web. 07 Feb. 2013. <http://www.bipm.org/en/scientific/mass/prototype.html>.
2. Bernstein, Matt A., and William A. Friedman. Thinking about Equations: A Practical Guide for Developing Mathematical Intuition in the Physical Sciences and Engineering. Hoboken, NJ: Wiley, 2009.
3. Rooney, Anne. The History of Mathematics. New York: Rosen Pub., 2013.
4. Batlle, Caravaca Ignacio., and Martí Joan. Tous. Carob Tree, Ceratonia Siliqua L. Rome, Italy: IPGRI, 1997.
5. Turnbull, Lindsay A., Luis Santamaria, Toni Martorell, Joan Rallo, and Andy Hector. "Seed Size Variability: From Carob to Carats." Biology Letters 2.3 (2006): 397-400.
6. Untracht, Oppi. Jewelry Concepts and Technology. Garden City, N.Y: Doubleday, 1982.
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