The unit of weight measurement for diamonds and precious stones in general is the Metric Carat which is equivalent to 0.200 grams.
But not even such a small unit is accurate enough for something so precious. Even in relatively inexpensive diamonds, fractions of a carat are equivalent to differences of hundreds of euros, which is why in the diamond industry the weight of gems is measured up to one thousandth of a carat and then rounded up to the hundreds, commercially called “point”.
Example: a 0.75 carat gem will be commercially described as “a 75 points” The term carat derives from the Arabic “qirat” or carob seed which was used in antiquity as a unit of measurement, thanks to the constant shape and weight of its seeds.
The price of the diamond, which is expressed per carat, never increases mathematically in relation to its weight. So a two-carat diamond does not cost twice as much as a one-carat stone of the same quality but much more. We must not confuse the carat for measuring the weight of gems with the “karat gold title”. The “carat title” indicates the purity of the gold, for example: Pure gold is described at 24 Kt (carat), while the description 18 Kt (eighteen carat) means that there are 18 parts of gold and 6 parts in the alloy of other metals.