Karats and carats both come from the same word source, carob, a tree with edible pods - better known as an alternative to chocolate. A poor one, in my opinion.

Karat is used to indicate gold content with 24K representing the pure metal. As pure gold is too soft for jewelry, it is alloyed with other metals for greater durability. A jewelry item is not considered gold unless it is at least 10K (USA), 9K (UK and Canada) or 18K (Italy and France). In some countries, the amount of gold is expressed as percentages or as parts of 1,000. So 18K gold would be 18/24 = 3/4 or 75% gold or 750/1000 = 750 parts.

Long ago, the weight of carob seeds were used as a gemstone weight reference (carat) to aid transactions. They were fairly uniform in size but the practice was likely subject to abuse by unscrupulous dealers who would use slightly different sized carob seeds to their advantage. Since the early 1900's, the weight of a carat has been standardised to one-fifth of a gram or 200 milligrams.

Mae West, the sultry and witty American movie star of yesteryear loved diamonds passionately and declared, " I never worry about diets. The only carrots that interest me are the number of carats in a diamond." More gemstone related quotations can be found here.

Photo credit : Sarahleeab in Flickr

References
Carat Wikipedia
Victoria Finlay (2006). Jewels : A secret history. Ballantine Books.
Antoinette L Matlins & A.C. Bonanno (1998). Gemstone Press.
New Scientist, 9 May 2006 "Did carob seeds allow shady diamond deals?"
Mae West quotations

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Original Post by THE BEADING GEM