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A Measure Of Things – Part Five


As an unreconstructed male, I have only ever had a very passing interest in jewellery and the terminology applied to describing the qualities of the bijou leave me nonplussed. I don’t really know my 18 carats from my 9. What do these descriptions signify? And what exactly is a carat?

It all started with the edible pods of Ceratonia silique or the Carob Tree. Traders in the Middle East weighed gold and gemstones using carob seeds in the belief that of all the seeds available, the carob was the most uniform in mass. In reality it was no more standard in size and weight than any other seed but let’s not spoil a good story. The solidus was introduced into widespread circulation by the emperor Constantine. It was almost pure gold and weighed around 4.5 grams or the equivalent of 24 carob seeds.

The fallacy that all carob seeds were uniform may have meant that you might have got a good bargain or were ripped off but at least it provided the etymological root for the unit of measure we now associate with gold and diamonds. The Greeks called the seed keration and this term was modified by the Arabs to qirat. It appeared in Italian as carato and first appeared in English sometime in the 15th century as carat.

Each country had its own standard for a carat which must have been very confusing for traders. In Cyprus a carat was 187 milligrams in weight whilst in Livorno it was 215.9. In London the carat was set originally at 205.409 milligrams but in 1887 it was adjusted to 205.303 milligrams. So confusing was the situation for international traders and consumers of jewellery that there was a demand for international standardisation.

This task fell to the General Conference of Weights and Measures which had been established in 1875 and met every 4 to 6 years in Sevres in France to thrash out agreement on internationally accepted units of measure. It was the Fourth Conference meeting in 1907 that turned its collective minds to the question of the carat and they concluded, not unreasonably, that what was good enough for the Romans was good enough for them. A carat or, more precisely, the metric carat was set at 200 milligrams, a figure that can easily be subdivided into tens and hundreds. Pure gold was 24 carats.

Talking of gold, the designations that apply to jewellery saying that it is 9 carat or 18 carat gold merely detail the amount of pure gold in the piece. 24 carat is as high as you can go – this is pure gold. 22 carat gold has 22/24ths of pure gold or around 91.6% whereas 18 carat has around 75% and 9 carat around 37.5%. The higher the carat rating the more expensive the piece but ironically, the lower the gold content the stronger and more durable the metal is. When you are accused of penny-pinching for going for the lower carat, just remind your beloved that it will be harder wearing. It might just work!

When it comes to diamonds, a carat is used to describe how much it weighs and this has been the case since around the 1570s. Each carat can be subdivided into hundredths, known as pointers in the jewellery trade. When used to describe gems, a carat has the same value as it has when applied to gold – 200 milligrams. A one carat diamond weighs 200 milligrams whereas one described as a 0.5 carat diamond will weigh 100 milligrams, one that is 0.25 carat, 50 milligrams – you get the picture. Its carat, together with clarity, colour and cut, goes a long way to determining its value.