carob carat diamond
The story of how the Carob became the Carat. In ancient days before our current set of weights and measurements had been arrived at traders around the Mediterranean needed to be able to judge the weight of a given item in order to decide its value, and they discovered that the seeds of a tree which grew very commonly in the area were very consistent in size. They were easy for traders to source and therefore these Carob seeds could be used in one side of a set of simple balance scales to measure the weight of the gemstone on the other side. Hence the carob became a unit of measurement for gemstones. The latin name for the Carob tree is Ceratonia Selequia, and in Greek  'Keration', this shortens to 'carat' quite easily.

 'The carat (ct) (not to be confused with the karat, sometimes spelled carat, a unit of purity of gold alloys), is a unit of mass equal to 200 mg (0.2 g; 0.007055 oz) and is used for measuring gemstones and pearls. The current definition, sometimes known as the metric carat, was adopted in 1907 at the Fourth General Conference on Weights and Measures and soon afterwards in many countries around the world'. Wikipedia

carob bean carat diamond

We found a Carob tree whilst on holiday and brought some pods home so that we could test how close to a carat they are, and it turns out that although they do vary (as has been found in scientific tests), they are pretty useful... more so than trying to judge a Diamond's weight by eye alone, and particularly so with the older more irregular cuts. In our quick experiment with this old 1.02ct Diamond the first chosen Carob seed weighed 1.01cts which we regarded as a reasonable success. We're not sure that we'll be using this method very often, but if you'd like to read more on the relationship between the Carob and Carat, the article listed below describes the results of tests which have been done to test their reliability.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1686184/

https://en.wikipedia.org/wiki/Carat_(mass)