The weight of a diamond is given in its carat equivalent. One carat is 0.2 grams and is divided into 100 points, i.e. 100 points equals 1.0 carat. The carat weight of a stone does not itself set the value of a stone; this depends heavily on the other three variables that exist; Colour, Clarity, Cut.
When choosing the perfect stone, it is a good idea to visualise the finger the ring is going on to ensure the proportions are correct. You should also think about the style of the setting and of course your budget.
One important factor to bear in mind is that the carat weight does not determine the value of the diamond. The other ‘C’s also influencing the value are Clarity and Colour. When diamonds are mined, large gems are discovered much less frequently than small ones, which makes large diamonds much more valuable. In fact, diamond prices rise in proportion to the carat weight. As such, a 2-carat diamond of a given quality is always worth more than two 1-carat diamonds of the same quality.
The above chart shows the diamond carat weight on the top row,
with the according approximate millimetre size of each stone beneath.